Mahmoud Ahmadinejad has been re-elected as president of Iran in a resounding victory, the interior minister says.
He won some 62.6% of the vote in an election marked by a high turnout of 85%, official figures show.
Supporters of pro-reform candidate Mir Hossein Mousavi have cried foul and clashed with riot police in Tehran, despite a ban on public protests.
Iran's Supreme Leader congratulated Mr Ahmadinejad on his win, and urged his rivals against "provocations".
In a statement, Ayatollah Ali Khamenei praised the high turnout and described the count as a "real celebration" and called for calm in the aftermath of the result.
John Simpson Reporting from Tehran
A crowd of about 3,000 attacked the police, some of whom were on motorbikes, which they set on fire.
The sky was thick with black smoke. Police attacked the crowd with sticks and maybe teargas.
I didn't expect to see people turning on the secret police. We were filming when we were surrounded by angry secret policemen. The crowd turned on them and chased them off.
I suspect we are not looking at a revolution but there is serious anger.
It all depends on how the government responds - if they use violence, that could inflame the situation.
"Enemies may want to spoil the sweetness of this event... with some kind of ill-intentioned provocations," the ayatollah said.
Mr Mousavi has also claimed victory, calling the result a "dangerous charade", as his supporters vowed to appeal for a re-run.
But observers say this would have little chance of success.
Riot police have used batons against a crowd of about 3,000 supporters of Mr Mousavi, some of whom were wearing his campaign colour of green and chanting "Down with the dictator", news agencies say.
Correspondents say this is the worst public violence for a decade in Tehran.
Four police motorbikes were set on fire near the interior ministry, the BBC's John Simpson in Tehran says.
The authorities had earlier sealed off Mr Mousavi's campaign HQ, preventing his supporters from holding a news conference.
Interior Minister Sadeq Mahsouli said that any demonstrations needed official permission, and none had been given.
The AFP news agency quoted a senior police official as saying: "The time of dancing and shouting is over."
One opposition newspaper has been closed down and BBC websites also appear to have been blocked by the Iranian authorities.
However, Interior Minister Sadeq Mahsouli said his share of the vote was 33.75%.
Danger of 'tyranny'
Mr Mousavi, a former prime minister, dismissed the election result as deeply flawed.
"I personally strongly protest the many obvious violations and I'm warning I will not surrender to this dangerous charade," the Reuters news agency reported him as saying.
"The result of such performance by some officials will jeopardise the pillars of the Islamic Republic and will establish tyranny."
Mr Mousavi had said there was a shortage of ballot papers and alleged that millions of people had been denied the right to vote.
His election monitors were not allowed enough access to polling stations, he added, saying he would deal seriously with any irregularities.
The head of the Committee to Protect the People's Votes, a group set up by all three opposition candidates, said the group would not accept the result, alleging fraud.
They have asked Iran's Guardian Council - a powerful body controlled by conservative clerics - to cancel the results and re-run the elections. A second opposition candidate, Mehdi Karroubi, declared the results "illegitimate and unacceptable".
Our correspondent says the result has been greeted with surprise and with deep scepticism by many Iranians.
The figures, if they are to be believed, show
Mr Ahmadinejad
winning strongly even in the heartland of Mr Mousavi, the main opposition contender.
The scale of Mr Ahmadinejad's win means that many people who voted for a reformist candidate in the previous presidential election four years ago have apparently switched their votes to Mr Ahmadinejad, he adds.
Police presence
BBC Iranian affairs analyst Sadeq Saba says the result means that hope for peaceful reform in Iran may die for a long time.
There had been a surge of interest in
Iran's presidential election,
with unprecedented live television debates between the candidates and rallies attended by thousands.
There were long queues at polling stations, with turnout 85%.
Remnants of the campaign are already being removed
Four candidates contested the election, with Mohsen Razai and Mehdi Karroubi only registering about 1% of the vote each.
President Ahmadinejad draws support mainly from the urban poor and rural areas, while his rivals have support among the middle classes and the educated urban population.
Iran is ruled under a system
known as Velayat-e Faqih, or "Rule by the Supreme Jurist", who is currently Ayatollah Ali Khamenei.
It was adopted by an overwhelming majority in 1979 following the Islamic revolution which overthrew the autocratic Western-backed Shah.
But the constitution also stipulates that the people are the source of power and the country holds phased presidential and parliamentary elections every four years.
All candidates are vetted by the powerful conservative-controlled Guardian Council, which also has the power to veto legislation it deems inconsistent with revolutionary principles.
What do think of the election results so far? How important are the elections for Iran's future? You can send us your views using the form below:
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.
BBC NEWS CHANNEL
E-mail this to a friend
Printable version
BBC News
Bookmark with:
What are these?