Police clash with supporters of Mir Hossein Mousavi in Tehran
Mahmoud Ahmadinejad has defended his "completely free" re-election as Iran's president, amid violent clashes on the streets over claims of election fraud.
Mr Ahmadinejad condemned the outside world for "psychological warfare" against Iranians during the election.
Thousands have protested against the result, burning barricades on the streets of Tehran and clashing with police, who responded with tear gas.
Reformist candidate Mir Hossein Mousavi urged his supporters to avoid violence.
Mr Ahmadinejad is due to hold a news conference on Sunday, followed by a huge victory rally.
'Down with the dictator'
Speaking on national television, Mr Ahmadinejad praised the Iranian people for choosing to "look toward the future" rather than returning to the past.
President Ahmadinejad addresses the nation
"This is a great victory at a time and condition when the whole material, political and propaganda facilities outside of Iran and sometimes... inside Iran, were total mobilised against our people," he said.
He blamed "foreign media" for instigating a "full-fledged fight against our people".
"Nearly 40 million people took part in a totally free election," he said.
BBC Iranian affairs analyst Sadeq Saba says the margin of the win was so wide that it made a lot of people suspicious.
He says given the high turnout, it was expected that Mr Mousavi would have won at least half of the votes.
Jon Leyne Reporting from Tehran
President Mahmoud Ahmadinejad has always seen himself as more than just a politician. Sometimes he speaks, and is treated, more like a seer prophesying the death of capitalism and liberal democracy. With this victory, however secured, he will feel emboldened in this global vision.
At home, many Iranians will fear a clampdown on society and cultural life. Mr Ahmadinejad knows that many even within the political establishment oppose him, which might increase his tendency towards authoritarianism. All those young people who were out campaigning against him so recently will be nervous about their future, and even more disillusioned with the Islamic Republic.
Then there is foreign policy and the nuclear issue. How can US President Barack Obama open negotiations with a president whose legitimacy and human rights record is even deeper under question? It is a political earthquake that will shake Iran, and could shake the world.
Millions of people, especially the young, voted for change through the ballot box and by peaceful means and now there is shock and disappointment, our analyst says.
The official result gave Mr Ahmadinejad a resounding 63% of the vote against 34% for Mr Mousavi and brought the worst violence seen in Tehran for a decade, correspondents said.
The BBC's John Simpson saw secret policemen being attacked and chased away by protesters, which he says is extremely rare.
Some of the protesters in Tehran wore Mr Mousavi's campaign colour of green and chanted "Down with the dictator", news agencies report.
Four police motorbikes were set on fire near the interior ministry, where votes had been counted, our correspondent says.
One BBC News website reader, Xenia, went to Vanak Square in Tehran to express anger and said
police fired plastic bullets.
"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."
The BBC's Jon Leyne in Tehran says clashes continued late into Saturday evening and there are reports that a number of reformist leaders have been arrested.
Ayatollah's call
Mr Mousavi urged calm in his website statement.
"The violations in the election are very serious and you are right to be deeply hurt," he said.
"But I firmly call on you not to subject any individual or groups to hurt."
Mr Mousavi earlier said the election was a "charade".
"I personally strongly protest the many obvious violations and I'm warning I will not surrender to this dangerous charade.
"The result of such performance by some officials will jeopardise the pillars of the Islamic Republic and will establish tyranny."
Iran's Supreme Leader Ayatollah Ali Khamenei, who wields ultimate power, urged all Iranians "including yesterday's competitors" to support the re-elected president.
He described the count as a "real celebration", praised the high turnout of 85% and called for calm. "Enemies may want to spoil the sweetness of this event... with some kind of ill-intentioned provocations," the ayatollah said.
Our correspondent, Jon Leyne, 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.
Our correspondent says Mr Ahmadinejad will feel emboldened in his global vision - one that foresees the death of capitalism - while at home many Iranians will fear a clampdown on society and cultural life.
Surge of interest
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 on Friday, with turnout reaching 85%.
Four candidates contested the election, with Mohsen Razai and Mehdi Karroubi only registering about 1% of the vote each.
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.
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