A massive group of demonstrators stretching nearly 10 kilometres rumbled through Tehran's streets on Monday in support of pro-reform politician Mir Hossein Mousavi, as rising tempers resulted in the death of one protester.
The man was killed when a pro-government militia shot at a group of demonstrators that had set a small fire to a compound linked to Iran's Revolutionary Guard.
Two other protesters were reportedly injured during the shooting, which came as anger over vote-rigging allegations spread across the country in a series of anti-government rallies.
The protests also raged in other cities and show little sign of subsiding. It appears they have been at least partly organized using new social media tools like Twitter to bypass censors. Twitter delayed a "critical network upgrade" late Monday so Iranians could continue to use it.
In an apparent move aimed at quelling the countrywide anger, Iran's ruling clerics allowed footage of the marches to air on television and also said an investigation would be launched into the election results.
Meanwhile, pro-reform candidate Mousavi made his first appearance since losing the disputed election to Iranian President Mahmoud Ahmadinejad.
In defiance of a government ban, the protesters gathered in Tehran's Azadi, or Freedom Square, and chanted "Long Live Mousavi" as the leader addressed the massive crowd.
"This is not election. This is selection," read one English-language placard at the demonstration.
"Respect the people's vote!" Mousavi shouted to a crowd estimated at 200,000 through a loudspeaker.
"We must regain our trampled rights and stop this lie and stand up to fraud and this astonishing charade," he said. "Otherwise, nothing will remain of people's trust in the government and the ruling system."
By the evening, opponents to the government climbed to Tehran's rooftops, and cries of "Death to the dictator!" and "God is great!" could be heard around the city.
As outrage rippled through Iran, global leaders expressed concern over the rising tensions.
Speaking for the first time since the election result on Friday, U.S. President Barack Obama said he was "deeply troubled by the violence I've been seeing on TV."
While Obama said there was no way of immediately validating or falsifying the election results, he
praised the democratic demonstrations.
"The world is watching and is inspired by their participation, regardless of what the ultimate outcome of the election was," he said.
Britain, Germany and Canada have also expressed concern about the election.
Election probe
According to Iranian state television, Ayatollah Ali Khamenei has directed a high-level clerical panel, known as the Guardian Council, to examine allegations of election fraud.
"Issues must be pursued through a legal channel," state TV quoted Khamenei as saying.
On Sunday, Mousavi wrote a letter to the 12-member Guardian Council and also met with Khamenei, who is the most powerful figure in Iran.
Khamenei said he has "insisted that the Guardian Council carefully probe this letter."
The Iranian government declared Ahmadinejad the winner of the election with 62.63 per cent support, while Mousavi trailed with only 33.75 per cent of the vote.
Mohsen Milani, chair of the University of South Florida's Department of Government and International Affairs, said this is the first time that the entire legitimacy of Iran's electoral process has been questioned.
The election results sparked massive protests over the weekend, which observers say were the worst in more than a decade.
During the weekend protests, thousands of mostly young people clashed with police.
Police beat the demonstrators with clubs 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.
Experts say that Iran's ruling clerics have backed away from cracking down too hard thus far out of concern about a backlash.
Some believe that resentment against the government could swell into anger against the ruling theocrats.
"That sets you up for a tremendous split," said Jon Alterman, who heads up the Middle East program at the Center for Strategic and International Studies in Washington.
"It could be tremendously destabilizing because if the office of (Supreme Leader Ayatollah Ali Khamenei) is damaged, then the whole shape of leadership ... moves into flux."
With files from The Associated Press
