Tehran was a tinderbox tonight after government paramilitaries started shooting during a huge public protest against last Friday's disputed re-election of President Mahmoud Ahmadinejad.
Members of the Basij, a force of young Islamic hardliners, killed one demonstrator and wounded several more when their building was attacked, after tens of thousands of demonstrators held a rally against election fraud in defiance of a government ban.
In another incident, a witness told The Times how she watched from her car as riot police on six motorbikes opened fire on youths walking under a bridge after the rally.
"The riot police started shooting them with big guns," she said. "It wasn’t like the films where there is just a small hole — the shooting was blowing off hands, limbs. It was terrrible, terrible."
Gunfire was heard in at least three other districts of the Iranian capital. The Ministry of the Interior was rumoured to have authorised the use of live ammunition as the regime struggled to maintain control. Supporters of the defeated candidate, Mir Hossein Mousavi, fought running battles with the police and Basiji, who have flooded into Tehran.
Mr Mousavi emerged from hiding for the first time since the election results were announced to address the rally. He told the crowd packed into Azadi (Freedom) Square: "God willing, we will take back our rights."
Newly emboldened, the protesters brought out their green ribbons and bandanas and chanted: "Mousavi we support you", and "We will die, but retrieve our votes".The show of strength boosted Mr Mousavi’s supporters, who have called for another rally and a general strike tomorrow.
The regime showed its first sign of alarm when Ayatollah Ali Khamenei, the Supreme Leader, who backed Mr Ahmadinejad’s re-election, instructed the Guardian Council of 12 senior clerics to investigate allegations that the election was rigged. However, the move was seen widely as a ruse to buy time.
The regime’s violent response to the most serious public protests since the student riots of 1999, and the biggest political crisis in the Islamic Republic’s 30-year history, has provoked a growing international backlash.
Gordon Brown warned Iran that the way it responded to legitimate protests would affect its relations with the rest of the world. David Milliband, the Foreign Secretary, expressed concern at "what seems to be state violence against its own people".
The US State Department said it was "deeply troubled" by the violence and Ban Ki Moon, the UN Secretary-General, urged the regime to respect the "genuine will" of the Iranian people.
"This is a turnaround for Khamenei, and it’s an embarrassing one," said Professor Ali Ansari, the director of the Iranian Institute at St Andrews University.
Today was the first time that Mr Mousavi had been seen in public since the election, as the authorities clamped down hard on protests, vowing to crush any "velvet revolution" in Iran.
Several hundred reformists have been arrested, including the brother of former reformist president Mohammad Khatami, prompting scuffles between relatives and police outside Tehran’s revolutionary court.
Baton-wielding police and militia have battled to drive back elusive flash mobs of demonstrators. Reformist newspapers have been barred from publishing, and the internet and mobile phone networks have been intermittently blocked.
Dozens more protesters were arrested overnight, as police and militia stormed the campus at Tehran’s biggest university after about 3,000 students began a dormitory demonstration, chanting "Death to the dictator".
Tehran University was the site of short-lived clashes during student-led protests in 1999, and is one of the nerve centres of the pro-reform movement.
Anger has been fuelled by comments made by the winning side. Mr Ahmadinejad told his supporters at yesterday’s giant victory rally that losing the presidential election was no worse than losing a football match, and that Mr Mousavi and his supporters should get over it.
With most of the foreign journalists allowed into Iran to report on the election due to leave today, there are fears that the crackdown may intensify. One death was reported early today, of a student protester in Shiraz, southern Iran, but the fatality has yet to be confirmed.
Iran’s powerful 12-member Guardians Council said today that it would rule on the candidates’ complaints within 10 days, providing a breathing space for the embattled regime.
With the chairman of the Guardians Council openly backing Mr Ahmadinejad before the election, the chances that it will overturn the result are thought to be remote.
Any finding of electoral abuses would further weaken Mr Ahmadinejad’s dented credibility at home, where his critics have already begun to speak out more clearly.
Mr Khatami, his predecessor, called for the election results to be annulled and a new vote held. He added that he was going to join the opposition rally in Tehran.
Iran also faced a growing backlash abroad over the election result. EU foreign ministers plan to issue a joint statement backing an election inquiry to clarify Mr Ahmadinejad’s win, and Chancellor Angela Merkel of Germany said that there were signs of vote irregularities.
Both the German and the French foreign ministers have summoned their respective Iranian ambassadors for a dressing-down.
"The actions of the Iranian security forces are completely unacceptable," said Frank-Walter Steinmeier, the German foreign minister.
Britain and America have taken a more cautious line, hoping to encourage Iran to end its international isolationism and rejoin talks over its controversial nuclear programme.