There has been widespread global outrage after twin rush hour attacks on Moscow's Metro network killed and injured dozens of people.
Here is a selection of quotes from around the world.
RUSSIAN PRESIDENT DMITRY MEDVEDEV
We will continue the fight against terrorism unswervingly and to the end.
RUSSIAN PM VLADIMIR PUTIN
Today a crime with horrible consequences and heinous in nature was committed against peaceful citizens in Moscow...
I am confident that the law enforcement agencies will do everything to trace and punish the criminals. The terrorists will be destroyed.
UN SECRETARY GENERAL BAN KI MOON
The secretary general strongly condemns the twin suicide bombings that took place in Moscow's subway this morning, causing the tragic loss of many innocent lives and injuries to many other people.
The secretary general is confident that the Russian authorities will bring to justice the perpetrators of this heinous terrorist attack.
The secretary general extends his deepest condolences to the families of the victims, and to the government and people of the Russian Federation.
UK PRIME MINISTER GORDON BROWN
There will never be any justification for acts such as these.
I've written to President Medvedev this morning to send our condolences to the victims and their families and I pay tribute to the Russian emergency services and the people of Moscow as they have responded to this appalling attack.
US PRESIDENT BARACK OBAMA
I send my deepest condolences to the people of Russia after the terrible loss of life and injuries resulting from the bombings on the Moscow Metro.
The American people stand united with the people of Russia in opposition to violent extremism and heinous terrorist attacks that demonstrate such disregard for human life, and we condemn these outrageous acts.
My thoughts and prayers go out to those who lost loved ones, and I wish all who sustained injuries a successful recovery.
FRENCH PRESIDENT NICOLAS SARKOZY
Russian authorities can be assured of France's total solidarity as they confront this cowardly and horrible act.
GERMAN CHANCELLOR ANGELA MERKEL
In the name of the Federal Republic of Germany, I should like to express my condolences at this terrible crime. Things must be clarified, but we will support Russia as far as possible.
This kind of violence, directed against the civilian population, is totally unacceptable and we are mourning with the Russian people.
NATO SECRETARY GENERAL ANDERS FOGH RASMUSSEN
On behalf of Nato, I strongly condemn the terrorist attacks in Moscow today.
There can be no justification for these kinds of attacks on innocent civilians. Nato remains committed to co-operating with Russia in the fight against international terrorism
EU PRESIDENT HERMAN VAN ROMPUY
I am deeply shocked by the terrorist attacks perpetrated this morning in Moscow's Metro that killed dozens of innocent people and left many others injured.