Flags with black ribbons have been flown at half-mast in Moscow to observe a day of mourning for the victims of Friday's underground bomb blast.
Thirty nine people are now known to have been killed in the explosion, and more than one hundred still remain in hospital.
Police say they do not yet have any clear leads in their investigation and are questioning survivors for clues.
Security has been stepped up across the underground network, with increased patrols and checks on passengers.
Did you witness the explosion? What is your reaction to the blast?
This debate is now closed. Read a selection of your comments below.
The following comments reflect the balance of opinion we have received so far:
It is quite a pity that nowadays it is innocent civilians that dye for political conflicts. Now, the Russian government will not negotiate with rebels in Chechnya to find out the problems. More terrorist attacks will continue.
Nelson Rotsaert, Belgium
As someone who lived in Moscow for a year, it was horrible to receive an email from my dear friend Masha which expressed her horror and deep sadness about the recent explosion. All humans seem to be equally impacted by terrorist acts - regardless of whom they are or where they are from. Humans were not intended to carry such deep seeded animosity toward one another. I am praying for peace!
Katya, Phoenix, USA
I have been to Moscow and believe it to be one of the most amazing places I have ever been. Having experienced the Metro, I understand the crowds that fill it regularly. I'm shocked that this has happened and my thoughts are with those who experienced this tragedy.
Katie Hassell, Warwick, England
I was on a college trip staying in the Ukraina hotel in Moscow when I heard about the bomb blast. It is strange to think that we had organised to visit the Metro that afternoon and travel on the same line that the explosion ripped apart. As a Westerner who watched the attacks of 9/11 on TV, it was even scarier to be that close to terror with no control over what was happening. My heart goes out to those families who lost loved ones in this attack.
Tom, West Sussex, England
It's horrible to read the news of that kind. I would like to give my support for those who had relatives in the train. The easiest thing is to instantly blame the Chechens. One should know that most of the Russian people could not even distinguish between Chechen and other Caucasian nations.
Kirill, Paris, France
This has affected people a lot. I don't mean that since it happened we are afraid to go out, but it seems that people are quite nervous about it. We try to behave normally, but the fear of this tragedy is stuck in the air. It is quite horrible at the moment.
Ekaterina, Moscow, Russia
The only thing which was not very clear and is not clear now - why everybody decided it was a Chechen terrorist who detonated the bomb. Maybe it was and maybe it was not. There is no clear evidence. Is it not quite strange that such things happen just before the elections? Now Putin can do everything he wants and say this is for the good of the country and against terrorism.
Margo, St. Petersburg, Russia
I have read some comments here where people refer to diplomatic solutions in Chechnya; but now a Saudi warlord has gained control of the terrorist element there it means the fanatics are firmly entrenched. Putin will never pull the troops out under such circumstances. With this type of terrorist in place there can be no diplomatic solution. I'm no admirer of Putin but he's making the best job of an impossible situation. The thing he must do now though, is set in place security measures so that Russian citizens can go about daily life safely.
I am 16 and at the time of the disaster me, 43 other students and 3 teachers were on a college trip and staying in the Hotel Ukraine. It seemed almost unreal to be physically in such close proximity and all of our group were disturbed by the sheer unpredictability of the attack. It bought home to us that such an event can literally happen anywhere, at any time, and to anybody. My thoughts are with all Muscovites who were involved or whose loved ones were involved. For me it showed how real terrorism truly is.
Justin Pickard, Horsham, UK
I live in the south of Russia, and am closer to Chechnya than Moscow. Last summer several blasts happened at the bus stations in Krasnodar. It can happen in any city and on any public transport. We have to live in fears and hope that we managed to avoid that terror.
Elena, Novorossyisk, Russia
It was a terrible act of violence on the Moscow subway, but what about the incidents that happen on a daily basis in Chechnya that aren't ever shown on our TV's? If the world saw what's happening to the Chechens there, the Moscow blast would seem very different. The Russians need to get out of Chechnya.
Knight, New York
It is so sad to read about the death of 39 innocent Russian citizens. I share the grief of their families.
Russian should think about policy of their president, this attack was consequence of Mr Putin's behaviour towards Chechnya. They shouldn't vote for him in the forthcoming elections. If they want peace they should guarantee peace and freedom for all Chechens.
Mariusz Gajda, Warsaw Poland
I studied abroad in Moscow about a year ago, during the theatre siege. That was a horrific event, and it saddened, but did not frighten all of the international students on exchange with me at the time. This, however, is even more horrific. The Metro is the lifeline of Moscow. This attack is an attack of everyone in Moscow. I hope that the situation can be improved and I hope to God that the Muscovites may live in peace and prosperity. I also hope this doesn't prevent foreigners from going to Russia. If nothing else, this has made me more intent on returning to the country.
Joe, Beloit, Wisconsin, USA
In China the entrances to ALL bus and train stations have x-ray machines. Even if you are only going in to buy a ticket or meet someone, ALL your bags and luggage will be x-rayed. It seems to work quite well and it doesn't really slow things down too much. Maybe this is what is needed.
Chris Hall, Weymouth, UK
I was really shocked about the act of terrorism and feel really sorry for the citizens of Moscow. I wish it had never happened and think the only way of working this problem out is for Russian troops to be withdrawn from Chechnya to stop the violence. Diplomatic solutions should also be found.
Talgat, Taraz Kazakhstan
Terrorism, I believe, is a crime against humanity, against the very fabric of humanity. But the methods adopted by some counties to fight terrorism are abetting it more than eradicating it. The most important step is to get to the root causes that make people behave in this callous manner.
M. Bashir Bharadia, Mombasa, Kenya.
I'm 22 and a native Muscovite. And it's a horrible feeling - a constant apprehension of impending danger, like a Damocles sword, a feeling that none of us will never ever be safe again. I'm now beginning to realise what my granny felt during the WWII. Perhaps this blast will finally make our government get up and DO something to protect the people (instead of thinking about how to hog yet another million for themselves).
Nadia Koretskaya, Moscow, Russia
Security on the Metro consists of police picking out non-Slav-looking people out of the crowd and checking their identity documents. This only gives the police a false sense of security. A "peace process" in Chechnya isn't likely going to stop terrorism here. It will take at least a generation for it to stop, in an optimistic scenario.
David, Moscow, Russia
What a nightmare! My mom was on that train. She takes it every day to get to work. Fortunately she didn't suffer any physical injuries and has been treated for shock. I don't know who is to be blamed. I pray for the victims and wish strength of mind to their relatives. Tonight I'm leaving Melbourne for Moscow.
Max, Melbourne, Australia
I used to take this line twice a day for quite some years before I moved my house. In rush hours it's actually quite crowded in both directions. The thing about Moscow metro that London commuters are not familiar with is that people treat privacy differently and will not politely wait for the next train to come or ask "can you move inside the train please" They rather push others to catch this train, even though the next one is coming in two minutes and there is little space to move in. Therefore it's pretty much standard when you have up to 300 people in every car and you travel jammed so much that you can't turn around. Therefore the blast caused so many casualties. A husband of my wife's friend went to work this morning and still did not show up at home - it's nearly midnight in Moscow. She knows he takes this line to go to work. He is not in the list of those who are in hospital. A list of those who died is still not available. He left his mobile at home. We all pray he is having late drinks with friends and will come soon.
Sasha, Moscow, Russia
Despair. It is what all of us feel here now.
Maria Znova, Moscow, Russia
I used to use this route to go to work. Fortunately for me my workplace has moved. I wish to offer my heartfelt condolences to the victims and their families. What gets me is that within a relatively short period of time Putin makes a statement blaming Maskadov and his forces, no evidence, no witnesses, no proof offered and even more unfortunately none required by the Russian population. Obviously if the great one makes such a statement it must be true.
James, Moscow, Russia
This is the line I use to go to work. It is a miracle that I had gone through earlier. This section is quite deep, just where the train goes underground after crossing the Moscow River. The train usually stops here for a few seconds before it heads into the centre of Moscow. This is probably the busiest line in Moscow, heading right into the centre. Up to 300 people fit in each carriage and often in the morning people cannot squeeze standing room. I can't imagine the carnage. This is a nightmare.
Martin Gee, Moscow
I live in Moscow and use the Metro every day. My relatives are studying near Paveletskaya station. It's a shock.
This is the same Metro station that I used for over two years to go to and from work, until I moved to Switzerland this week. What a nightmare.
The explosion amply demonstrates the bankruptcy of the current policy to contain terrorism. Today also marks the threshold when terrorists in Russia encroached on the frailest underbelly of any city - its transportation. It is impossible to withstand such bloodily blatant acts with conventional security methods. Indeed, the only way out is equipping Moscow Metro stations with screening machines.
Artemenkov Andrey, Moscow, Russia
Hearing the news about attacks almost every day is very unpleasant. Every evening when I watch the news there is always news about a terrorist attack of some sort. President Putin should do all that is in his power to stop the constant terror of commuting to and from work. People are scared to go to the theatre and to movies. The life in Moscow today is very uncomfortable because of constant fear.
Dima, Minsk, Belarus
I am originally from Moscow. I am very sad and angry to hear this news. Sad for the victims and angry because someone thinks that by killing people he can advance his lost cause. I call on Chechen terrorists to seek a diplomatic solution to whatever grievances they have.
Eugene, New York, USA
I get to my office by metro to Paveletskaya station and today I've managed to escape the most terrible thing in my life just because I overslept! People use the underground every day and now everybody is afraid of going there. I don't feel like risking my life for the sake of the terrorists' ambitions. Unfortunately, the latest blast attacks haven't been predicted by our security councils, however, nobody's to blame here except the people that has taken up terrorism as an effective means of frightening the country.
Vivian, Moscow, Russia
I remember the bomb explosions in London in 1992 (I was there at the time). It was terrible to know the innocent people being killed and wounded in the streets of a peaceful city. Now we have the trouble in Moscow. My opinion is that terrorism is a crime against humanity. The terrorist and especially their leaders and supporters are to be treated the same way as Nazi criminals. UN initiative is needed to declare all kinds of terrorism the crime against humanity. All countries are to unite in the struggle against the evil of terrorism. They do not just explode bombs; they try to explode the foundations of democracy. Remember the lessons of history: terrorists did a lot to destroy the appearing democracy in Russia at the beginning of the 20th century.
Andrei Sedin, Moscow, Russia
While any act of barbarity like this which kills innocent people is atrocious in any country, Putin needs to address the situation in Chechnya, with a diplomatic solution, like John Major and Tony Blair did with the fighting factions in Northern Ireland. Unfortunately as it's too dangerous for many journalists to report from Chechnya, we're really not getting to hear all sides of the argument.
Peter Wilson, Poland
I saw the breaking news on BBC World and was shocked to hear about another deadly attack. What amazes me even more, though, is that no Russian channel was airing the news as it occurred. There was a humour programme on the Public Russian Television (ORT) and another Moscow channel aired a talk show. I have relatives living near Moscow and I'm scared for them. Russian troops should be withdrawn immediately from Chechnya to stop violence.
Maxim, Kyiv, Ukraine
It's not true that there was no news about the blast on Russian TV. Moreover, the breaking news concerning today's explosion has been broadcast every 30 min, giving new information and telephone numbers of the hospitals to which the injured people have been taken. The city services work perfectly well. We've got some worries about the situation in Moscow, but we are not scared. There's no panic or hysterics.
I was born in Moscow and have lived there all my life. I am 23... Today's explosion took place a station away from the one near which I work. Fortunately I went to work by car today. It feels like we're living in wartime. Atrocities occur around us and nobody gets punished. In America, Bush has a colour-coded scale of alerts, puts armed guards on the planes of every airline and scans in people's fingerprints. And here? For a week after the explosion, we check the ID of people of Caucasian appearance on the metro, right? Anyone who says they are not afraid is a liar. I personally am afraid.
Viktor, Russia [translated from Russian]
I live in the apartment building located right above the subway station "Avtozavodskaya", this is the route I normally take to travel to the university. The ridiculous thing is that while there is a decent number of policemen present at every single subway station (in theory charged with safety and verifying suspicious passengers' identity), instead of looking for real potential troublemakers and terrorists they choose to pick the ones with likely fake registrations.
Aleksandra (Sasha), Moscow, Russia
A cowardly act by weak people...my girlfriend lives in Moscow and uses the Metro system a lot which makes me feel less than secure.
I was on this line only 3 months ago and it scares me to think of how this could have affected my life. I hope my children won't have to suffer from the scourge of terrorism.
Richard Freye, Charlton, London
For all that, they don't scare us and we're not afraid. The emergency services worked smoothly, there was no panic. Muscovites are alarmed but not frightened. You don't intimidate us! It didn't work. The men are at work. They're grim and they're mad. The only thing I'm afraid of is that if there are two or three more terrorist attacks like this we could see lynch mobs - something we just don't need. What if that Caucasian woman in the long coat is actually pregnant? It's easy to get it wrong at first glance. And how would you explain afterwards? That's what scares me.
Vitali Inozemtsev, Moscow [translated from Russian]
God what a nightmare! And people are already used to it - "proceeded in an orderly manner". That's what's scary.
Masha, Russia [translated from Russian]
Shock. I heard about the tragedy when I got to work. My hands are shaking and I can't work. We should sack the whole FSB [security police]. I was born in Moscow and I used to love the city where eight generations of my family lived, but now I hate it.
Lyudmila, Russia [translated from Russian]