Thousand of Iranians clashed with police in Tehran after the disputed re-election of President Mahmoud Ahmadinejad lead to the biggest protests in the country in a decade.
The brazen demonstrations, mostly carried out by young people, turned the city to chaos Saturday.
Police beat the demonstrators with clubs and smashed cars while officers on motorcycles chased protesters through the streets.
Some protesters retaliated by throwing stones at police.
Authorities have responded by shutting down cell phone services in Tehran and key social networking websites have been blocked.
The Iranian government declared Ahmadinejad the winner of this week's election with 62.63 per cent support, results disputed by the leader's main rival as tensions between the two sides boiled over in Tehran.
Ahmadinejad's opponent, reformist Mir Hossein Mousavi, trailed with only 33.75 per cent of the vote, despite what many experts believed would be a close election.
The final vote tally led to clashes between riot police and Mousavi supporters on the streets of Tehran Saturday.
Protesters, many wearing the green colours of the reform-minded Mousavi's campaign, set tires on fire outside the Interior Ministry, which ran Friday's elections, despite increased security around the building.
On one of Tehran's main streets, about 300 people formed a human chain chanting, "Ahmadi, shame on you. Leave the government alone."
Mousavi said he believed the election was rigged and threatened to challenge the results, which he called "treason."
"I'm warning that I won't surrender to this manipulation," said the Mousavi statement Saturday. "The outcome of what we've seen from the performance of officials ... is nothing but shaking the pillars of the Islamic Republic of Iran sacred system and governance of lies and dictatorship."
He warned "people won't respect those who take power through fraud" and called the decision to announce Ahmadinejad winner of the election a "treason to the votes of the people."
The reformist protesters are now at a crossroads: will they shrink back into a quiet anger or will they continue to battle Iran's powerful security forces as they did Saturday?
Global concerns of 'rigged' election
Canadian Foreign Affairs Minister Lawrence Cannon said he was "deeply concerned" about the reports of voter fraud and intimidation in the Iranian election.
"We're troubled by reports of intimidation of opposition candidates offices by (Iran's) security forces," Cannon said Saturday at an event with U.S. Secretary of State Hillary Clinton.
Cannon said Canadian embassy officials are monitoring the situation.
"Canada is calling on Iranian authorities to conduct fair and transparent counting of all ballots," he said.
Clinton took a softer approach, and said the U.S. was refraining from commenting on the election.
"We are monitoring the situation as it unfolds in Iran, but we, like the rest of the world, are waiting and watching to see what the Iranian people decide," Clinton said. "We obviously hope the outcome reflects the genuine will and desire of the Iranian people."
The election results complicate President Barack Obama's plans for the region, as he planned on starting diplomatic talks after the election.
Juan Cole, president of the Global Americana Institute and University of Michigan professor, wrote on his website Saturday that there is significant evidence suggesting the election was stolen.
A statement on Mousavi's website encouraged followers to resist a "governance of lies and dictatorship."
Mousavi asked Iran's supreme leader, Ayatollah Ali Khamenei, to intervene, but he refused.
In a statement delivered on state television, Khamenei called the result a "divine assessment."
The Iranian government appeared to be trying to shut the president's rival off from his supporters, as several pro-Mousavi websites were either blocked or hard to access.
Text messaging, one of Mousavi's primary ways to get his message out to supporters, has also been blocked.
The election results will do little to thaw relations with the international community. Iran has come under increasing pressure from the United States to curb its nuclear weapons program and Ahmadinejad has faced criticism for inflammatory comments in which he continually denies the holocaust.
The president has little control over Iran's foreign policy, given that most decision-making rests with ruling clerics headed by Khamenei.
