Increasing the level of confrontation, he said that Opposition leaders would be “responsible for bloodshed and chaos” if they did not stop further rallies in protest of last week’s disputed presidential election. He denied the Opposition’s accusations that the vote was rigged, praising the officially declared landslide for Mahmoud Ahmadinejad as an “epic moment that became a historic moment”. Ahmadinejad listened quietly in the audience.

Rendering the partial recount under way by the Guardian Council irrelevant, Khamenei said: “The Islamic state would not cheat and would not betray the vote of the people.”

Khamenei spoke for more than an hour and a half during Friday prayers to tens of thousands of people at the Tehran University. His sermon was broadcast over loudspeakers, and the crowds erupted repeatedly in roars of support. Opposition supporters had decided not to attend, and there was no sign of them there, or at a rally in the same area for the volunteer militia, the Basijis.

There was no immediate response from Opposition leader Mir Hussein Moussavi. No Opposition rally was planned for Friday. But on Saturday, a group of reformist clerics loyal to former President Mohammed Khatami plan to demonstrate against the election results.

Iranians were looking to Khamenei’s sermon for clues as to whether the authorities were prepared to bend to Opposition demands. But he showed no move toward conciliation.

Khamenei reached beyond Iran to criticise the “media belonging to Zionists, evil media” for seeking to portray Iran as divided and accused what he called arrogant Western powers, particularly Britain and the US, of hostile comments, saying they failed to understand Iranian society. He singled out Britain as the “most treacherous” of the Western powers, prompting the British Foreign Office to swiftly summon Iran’s ambassador in London to complain.

At a news conference after a European Union summit in Brussels, Prime Minister Gordon Brown said: “We are with others, including the whole of the European Union unanimously today, in condemning the use of violence, in condemning media suppression.”

In Washington, a Republican-backed initiative to condemn the Iran’s crackdown on the Opposition passed the House.

Khamenei instructed dissenters to pursue their complaints about the June 12 ballot through legal channels, insisting that the turnout, put at 85 per cent, proved that it was a reflection of the national will.

“Sometimes the difference between two candidates, he said, “is 100,000, 500,000, 1 million, so at that time there may be some doubts about cheating. But how can 11 million votes be replaced or changed?... This is a sign of God’s mercy for this nation. The fate of the country should be decided in ballot boxes, not on the streets.”

He refused to give in to what he called “illegal pressure” to annul the election. “If we break the law, we will have to do it in every election and no election would be immune,” he said.

“Flexing muscles on the streets after the election is not right. It means challenging the elections and democracy.”

It was not clear what role was being played by former Iranian President Ali Akbar Hashemi Rafsanjani, who supported Moussavi and is in a power struggle with Ayatollah Khamenei.