Iran's Supreme Leader today singled out Britain as the "most treacherous" Western power trying to destabilise the Islamic Republic as he stood firm in the face of this week's massive street protests, hinting at even tougher repression if the unrest does not stop.

In his first public appearance since President Mahmoud Ahmadinejad's hotly-disputed re-election, Ayatollah Ali Khamenei set the scene for further confrontations between his regime and the millions of demonstrators who have taken to the streets of Iranian cities in the past six days, by insisting the political establishment would not bow to such pressure.

Despite instructions from the Supreme Leader, Mir Hossein Mousavi, Mr Ahmadinejad's defeated rival, failed to attend Mr Khamenei's nationally-televised sermon to a vast crowd gathered at Tehran University for Friday prayers, and his supporters are planning to defy the authorities with another huge rally tomorrow. Mr Ahmadinejad, by contrast, sat at the front.

In the Iranian equivalent of a US President's Oval Office address to the nation, Mr Khamenei described the elections as "epic".

He said the huge turnout of 85 per cent showed how much the Iranian people trusted their political establishment, and the superiority of Iran's brand of religious democracy.

Mr Khamenei said foreign powers were now conspiring to ruin that achievement, and to destroy the Iranian people's trust in their political leaders, by suggesting the vote was rigged. He named Britain as the "most treacherous" of those countries, prompting roars of "Death to the UK" from the solid ranks of Basiji – Islamic volunteer militiamen – packed into the vast prayer hall.

The Islamic establishment would never commit "treason" by manipulating the vote, he said. "The legal mechanisms in our country do not allow cheating."

The 11 million-vote gap between Mr Ahmadinejad's tally and that of Mr Mousavi made a mockery of claims that the result was fixed, he suggested. If anyone had evidence to the contrary they should pursue it through legal channels.

Mr Khamenei demanded an end to the demonstrations. "I want to tell everyone these things must finish. These street actions are being done to put pressure on leaders but we will not bow in front of them," he said.

Without naming the three losing candidates who have challenged the election results, he ordered them to "open their eyes" and see behind the demonstrations "the enemy hands working, the hungry wolves waiting in ambush".

He added, with distinct menace: "Those politicians who somehow have influence on people should be very careful about their behaviour if they act in an extremist manner...This extremism will reach a sensitive level which they will not be able to contain. They will be responsible for the blood, violence and chaos."

Mr Khamenei also blamed the deaths, violence and vandalism of the past week on "ill-wishers, mercenaries and elements working for the espionage machines of Zionism and western powers".

The faithful, bussed to the prayer hall in their thousands, roared their approval. The speech is likely to have been received rather less rapturously on the streets outside - or in Western capitals.

In London, the Foreign and Commonwealth Office summoned Rasoul Movahedian, the Iranian Ambassador, to explain Mr Khamenei's comments. Mr Movahedian is due to meet Mark Lyall Grant, the FCO political director, later this afternoon.

The strength of Mr Khamenei's words - and his explict warning against further protests - does not bode well for tomorrow's mass rally in Tehran, which Mr Mousavi is due to address.

So far the Iranian security forces have not tried to prevent mass demonstrations and the Tehran governor, Morteza Tamadon, called for the rally to be called off.

Another of the defeated candidates, the reformist former parliament speaker Mehdi Karroubi, had urged his supporters to stay away from the Tehran prayers addressed by the supreme leader and instead join tomorrow's rally.

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