TEHRAN (AFP) - Seven people were killed when a massive opposition rally in the heart of Tehran against the re-election of Iranian President Mahmoud Ahmadinejad turned violent, state radio said on Tuesday.

Tires, dustbins and motorbikes were set ablaze by protestors as hundreds of thousands of Iranians took to the streets in a public outpouring of anger reminiscent of the days of the Islamic revolution in 1979.

State radio said at least seven civilians were killed when “thugs” attacked and vandalized government buildings at the end of the rally, which had been banned by the authorities as an illegal gathering.

“A military post was attacked with the intention of looting its weapons. Unfortunately, seven of our citizens were killed and a number of them injured,” the radio said.

One source told AFP that the emergency services department had information that eight people were killed, while the coroner's office said it had registered no deaths in connection with Monday's rally.

The mounting protests against Ahmadinejad's re-election in a vote his main defeated challenger Mir Hossein Mousavi has branded a rigged the election as a “charade” triggered the worst unrest in Tehran in a decade.

Iran is facing an international backlash over its crackdown against the opposition protestors and the election itself, which returned the combative Ahmadinejad to another four years in power.

United States President Barack Obama, who has called for dialogue with the Islamic republic after three decades of hostility, said he was “deeply troubled” by the violence.

In one incident late Monday, one man was reportedly shot in the head outside the local base of the Basij Islamic volunteer militia, which was set on fire.

Pictures showed armed men, wearing helmets and in civilian clothes, pointing guns at the crowds from the rooftop of the base.

In the face of the unrest, Iran's supreme leader Ayatollah Ali Khamenei has order a probe into the allegations of vote-rigging laid by former wartime premier Mousavi, who had declared himself the victor on polling day Friday.

The violence flared after Mousavi appeared in public for the first time since the election that highlighted deep divisions in Iran as it grapples with a struggling economy and a standoff with the West over its nuclear drive.

“God willing, we will take back our rights,” Mousavi shouted from the roof of a car amid a sea of hundreds of thousand of Iranians, young and old, who packed into central Tehran despite the ban.

One policeman said between 1.5 and two million demonstrators, some wearing the green of Mousavi's campaign color, had swarmed into central Tehran.

In his first public comments since the disputed election, Obama called on Iranian leaders to respect free speech and democracy, saying that “it is up to Iranians to make a decision about who Iran's leaders will be.”

“I think it's important that moving forward, whatever investigations take place are done in a way that is not resulting in bloodshed and is not resulting in people being stifled in expressing their views,” he added.

United Nations chief Ban Ki-moon has called for the will of the Iranian people to be “fully respected.”

European governments also complained about the tactics used against protesters and added their voices to US doubt over the election outcome, with the European Union calling on Tehran to launch a probe.

“The regime must address the serious questions which have been asked about the conduct of the Iranian elections,” British Prime Minister Gordon Brown said.

German Foreign Minister Frank-Walter Steinmeier described the action of the security forces as “completely unacceptable,” while French President Nicolas Sarkozy he wanted “full light” to be shed on the vote.