IRAN’S hardline leaders warned last night that they would crush dissent after opposition supporters protesting against their candidate’s defeat in disputed presidential elections clashed with riot police on the streets of Tehran.

In the Iranian capital’s most serious unrest for 10 years, thousands of liberals who claimed the election had been rigged vented their fury in running battles with police.

They fought officers armed with batons and stun grenades, set fire to police vehicles and threw stones at government buildings.

I saw police in camouflage uniforms and black flak jackets respond by firing the grenades from motorcycles into a crowd that chanted “Down with the dictator” and denounced what it called a stolen election.

In a stand-off near the interior ministry, which oversaw the count, opposition supporters formed barricades of burning tyres, sending plumes of smoke over the city. Helmeted police chased protesters who became detached from the main group and beat them with truncheons.

The crowds came on to the streets after the reformist candidate, Mir Hossein Mousavi, described the election as a “dangerous charade” that could bring tyranny to Iran and said he would never surrender.

A separate confrontation with supporters of President Mahmoud Ahmadinejad, the declared winner, ended in frenzied fist-fights.

Clashes continued late into the night and fires of rubbish burnt as thousands of militiamen armed with knives and truncheons swarmed through the city in motorcycle packs of about 20. Lorries laden with riot police roamed the streets.

Private polling of 5,000 voters conducted for the supreme leader, Ayatollah Ali Khamenei, and seen by The Sunday Times, suggested the reformist candidate would win at least 58% of votes across Iran. However, the official result gave him just under 34%.

Khamenei ordered all Iranians to accept the reelection of the president, whose vote was put at 63%. Surprisingly, Ahmadinejad was said to have defeated his main challenger and two others in their home towns, with turnout estimated at a record 85%.

Mousavi, who declared himself the “definite winner” immediately after the polls closed, later accused the authorities of blatantly fixing the election, which was not observed by international monitors.

He appealed to senior clerics to intervene, saying: “No one even imagined this much vote rigging and in front of the eyes of the world by a government which says it is committed to religious justice.” However, Khamenei, who refused to see him, called the results a “divine assessment”.

The result confounded the expectations of Iranian and western analysts alike and destroyed Mousavi supporters’ hopes of a “green revolution” that would herald greater freedom, more equality for women and less isolation from the West.

It was a setback to American hopes that an “Obama effect” could lead to a less confronta-tional government in Tehran, which is believed to be developing nuclear weapons.

The Israeli foreign minister, Avigdor Lieberman, said the result highlighted the need for action against Iran’s nuclear programme. Ahmadinejad declared the election “free and healthy” and said it was “the greatest achievement of the Iranian people”.

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• 'Was it real?' The US bailout nightmare
• Iranian opposition plays the long game