The protests in Tehran were widespread
Thousands of opposition supporters have clashed with police after President Mahmoud Ahmadinejad was declared the winner of Iran's presidential poll.
Secret police have been attacked, while riot police used batons and tear gas against backers of candidate Mir Hossein Mousavi, who called the results a "charade".
The official results gave Mr Ahmadinejad 63% of the vote and 34% for Mr Mousavi. Iran's Supreme Leader Ayatollah Ali Khamenei praised the high turnout of 85%, described the count as a "real celebration" and called for calm.
BBC News website readers in Iran have been sending their reactions:
XENIA, JOURNALIST, TEHRAN
I headed for Vanak Square, one of the main squares in Tehran, to express my own anger and frustration at the election results.
Fires were burning everywhere. The protesters had set fire to buses, motorcycles, litter boxes and just about anything
It was a scene of chaos and violence. The security guards had blocked all main roads and streets. Fires were burning everywhere. The protesters had set fire to buses, motorcycles, litter boxes and just about anything.
They were angry, extremely angry. They were shouting slogans, cursing the whole regime!
They felt deceived just as I did. My first vote after coming to my homeland- and most likely my last! We were angry at the extent of hypocrisy. Ten days of hard work, of campaigning, of joy and hope and alas, election day. And in seven hours the results were out, Ahmadinejad in the lead!
We are disgusted. The government, our government, had fooled us. We were just about sure that Mousavi will be the next Iranian president.
I don't live in Northern Tehran, I live in the city centre. I socialise with many people and I know that many of them only voted for the sake of change - and that definitely is far from four years of isolation, hatred and distrust.
And what do we get, us people, us who relied on this government again thinking it will be different this time around? We get beaten by batons. I was hit by one of the guards myself for filming them attacking the crowds. I was told by many that I was actually lucky for getting away.
They shot plastic bullets to disperse us. They fired tear gas to scare us away, but it would only be temporarily. We would hide in some corner and then re-emerge angrier than ever.
NOUROLLAH AJDARI, MANAGER, TEHRAN
I voted yesterday for Karroubi. I was critical of some of Mousavi's policies but I expected that he would win. Most of the people queuing were voting for Mousavi.
At first I did not want to vote because I did not think it would be a fair election. I was sceptical but I thought it would be a chance for change.
The figure announced for the number of votes polled for Mr Karroubi was ridiculous. There were more people working in Karroubi's campaign than the number of votes he is said to have received.
When I arrived at work people did not want to work. They were disappointed and angry. It was a very bad atmosphere.
I think the votes have been manipulated. It was coup which has been engineered by Ahmadinejad supporters.
I wanted to go out and see what is going on in the streets. I have heard that people are demonstrating. The people feel they have been betrayed.
I just wonder what the candidates can do? They have called it a charade but what can they do? I am astonished and bewildered. My colleagues were crying.
Now I do not know what is going to happen. I am sure there will be more sanctions and pressure from the international community.
ARYA DIBA, UNIVERSITY STUDENT, KURDISTAN PROVINCE
I did not vote, I think the system is corrupt and this result shows that it is corrupt. I am very disappointed with the outcome.
People are not happy with Ahmadinejad, some of my friends cried when they heard the results. It is a sad day for us.
At the moment there is an atmosphere of disbelief and people are thinking about how to express this. Everyone is just shocked.
All of my family voted for Mousavi, my father voted for the first time and there was a great turnout. There is a lot of support for Mousavi where I am in the Kurdistan region.
The margin is greater than expected and I have no trust in the system. There is not much that people can do at the moment, we need leadership.
UNHAPPY WITH THE RESULT
I am just shocked. I still cannot believe the results. There is the possibility of massive rigging. The result is quite unexpected in Iran. Everybody thought Mousavi would have a landslide victory. Unfortunately democracy for Iranians is a far-fetched ream. Today is the national mourning for democracy in Iran. With the existence of such a complicated power system in Iran and the supreme leader on top no reformation can take place in near future. Long live democracy.
Morteza, Shiraz, Iran
I know many people voted with high hopes to put Ahmadinejad out of office. Everybody is deeply disappointed. This election is obviously a fraud and proves there's no true democracy in Iran. No reform can be achieved by trusting this system of election.
Arya Diba, Tehran - Iran
I think the election is a fraud. My heart goes out to the Iranian people who want change.
Georgina, Brooklyn New York
This is evidently a coup. Under no conditions, Ahmadinejad can get such a vote. Even in the former elections, he won with a lesser vote while he was challenging Akbar Hashemi Rafsanjani who used to be an unpopular Mullah and people voted for Ahmadinejad just to get rid of Mullahs. The huge turn-out in this year's elections was mainly contributed by supporters of Mousavi. People who never vote came to vote for Mousavi not for Ahmadinejad. I am definitely sure that Mousavi is the actual winner of these elections and nothing will change my mind even if he himself admits defeat. Iran is administered by the most tyrannical regime and the system is just making fool of people.
Arash Aryan, London
I feel robbed, I feel abused. Today I truly realized that I'm living in a third world country. Today I learned that the worst thing in life is to think that you have a chance while you don't.
Milad, Mashhad, Iran
IN SUPPORT OF THE WINNER
I am an Iranian living in the UK. Yesterday I voted for Mr Ahmadinejad in Manchester and I saw many other Iranians voting for him. Apparently Mr Mousavi thinks too much of himself. It's either that or he thinks people can easily close their eyes and not see how much work Mr Ahmadinejad and his team have done for Iran over past 4 years. I hope Mr Mousavi and his supporters can finally recognise that most people don't want what they want.
It just goes to show that whatever the Western world has been saying about Ahmadinejad and Iran, they were wrong and the Iranian people have put their support towards Ahmadinejad regardless of what other would think of him, he is now confirmed president so live with it people!
Dilwar Hussain, London
Personally, I felt great excitement on hearing the victory of former president. He is supposed to be the one and only hero who is ruling the world. Heartlfelt congratulation to Mahmoud Amahdinejad
Durga Limbu, Kathmandu, Nepal