Iran's top legislative body, the Guardian Council, said a partial recount on Monday had disproved complaints of irregularities by pro-reform opponents, who said the count was inadequate and that only annulling the election would do.
Riot police beefed up their presence in the capital Tehran but there were no signs of major unrest late on Monday, in contrast to protests that erupted when Ahmadinejad was first declared victor of the June 12 vote.
State media say 20 people died in that violence which the government and opposition blamed on one another. Pro-government Basij militia and riot police broke up the protests.
"The secretary of the Guardian Council, in a letter to the interior minister, announced the final decision of the Council ... and declares the approval of the accuracy of the results of ... the presidential election," state broadcaster IRIB said.
The poll and its turbulent aftermath have exposed splits in Iran's political establishment and plunged the country into its deepest crisis since the 1979 Islamic Revolution.
But options for the opposition look limited now the election result has been officially upheld, after the recount of what the Council said was a random 10 percent of the vote.
Supreme Leader Ayatollah Ali Khamenei signalled on June 19 that mass protests would no longer be tolerated.There is little scope for more legal fights, and hundreds of opposition supporters have been detained, leaving protesters leaderless.
After dark, some people are still chanting "Allahu Akbar (God is greatest)" from their rooftops, mimicking tactics used during the 1979 revolution, but the nightly cries are weakening.
There has been talk of other forms of civil disobedience, including strike action, but these have yet to materialise.
Dossier Closed
"The Guardian Council statement was issued when it became convinced about the accuracy of the election," a Council spokesman said, adding that no irregularities were found.
"The dossier of the ... election has been closed today."
The recount system was not immediately clear, but state media said it had been spread over at least several provinces.
Opposition supporters say the vote was rigged to favour the hardline president over reformist rivals including Mirhossein Mousavi, who came second. Mousavi had rejected the idea of a recount and sent no representatives to watch it.
"The Guardian Council approval of the vote negates the possibility of an election re-run," Iran's English-language Press TV television station said,
Strains With West
U.S. Secretary of State Hillary Clinton said Iran's rulers were still likely to face internal opposition.
"They have a huge credibility gap with their own people as to the election process. And I don't think that's going to disappear by any finding of a limited review of a relatively small number of ballots," she said.
Asked if Washington would recognise Ahmadinejad as president of the world's fifth biggest oil exporter, she said:
"We're going to take this a day at a time."
Iran's rulers, locked in a row with the West over nuclear ambitions, have blamed the post-vote trouble on foreign powers rather than popular anger.
The protests strained relations with Britain in particular, which rejected accusations its embassy encouraged the unrest.
Iran had detained nine local embassy staff but freed five of them on Monday, saying the others would be kept for questioning.
"Iran's action, first the expulsion of two diplomats and now the arrest of a number of our locally engaged staff, is unacceptable, unjustified and without foundation," Prime Minister Gordon Brown told reporters in London.
