Clashes broke out between Iranian police and opponents of Mahmoud Ahmadinejad in Tehran on Saturday after the re-election of the hardline president.
Chanting "the government lied," thousands of protesters took to the streets, setting trash bins and tires ablaze.
Demonstrators pelted police with rocks and bottles as they accused Ahmadinejad of stealing the election from his reformist rival, Mir Hossein Mousavi.
Police fought back with clubs, including mobile squads on motorcycles swinging truncheons.
Violence broke out after election results showed a nearly 2-to-1 landslide for Ahmadinejad.
Mousavi declared himself the true winner of Friday's presidential race and urged backers to resist a government based on "lies and dictatorship."
According to government officials, Ahmadinejad won 62.6 per cent of the vote, while Mousavi garnered 33.75 per cent in a contest that appeared to divide urban against rural voters.
'Power through fraud' alleged
Mousavi did not appear in public but warned in a web message: "People won't respect those who take power through fraud."
The former prime minister, who has become popular with young voters in Iran, was not yet conceding defeat, and he claimed there were voting irregularities.
"I won't surrender to this manipulation," said a statement on Mousavi's website.
More than 470 people had sought to join the presidential race, but only Ahmadinejad and three rivals were cleared to do so.
After news of the election win, Iran's supreme leader, Ayatollah Ali Khamenei — who has final say on all state matters — thanked the people for their record 85 per cent participation and warned opposition candidates to avoid what he described as "provocative" behaviour.
"The chosen and respected president is the president of all the Iranian nation," Khamenei said in a statement read on state television, adding that all Iranians "must unanimously support and help him."
The presidential campaign reached a climax in the past few days with duelling rallies by supporters of Ahmadinejad and Mousavi that drew tens of thousands into the streets of Tehran.
Ahmadinejad maintained broad support among rural and working-poor voters, while Mousavi took much of the middle-class and urban vote.
The re-elected president will serve a four-year term.
U.S. Secretary of State Hillary Rodham Clinton said she hoped the outcome reflects the "genuine will and desire" of Iranian voters.
At a joint appearance with Clinton in Niagara Falls, Ont., Canadian Foreign Affairs Minister Lawrence Cannon said his country is "deeply concerned" about reports of irregularities in the election.