Iran's supreme leader, Ayatollah Ali Khamenei, issued a stern warning to opponents today to stay off the streets and denied claims that last week's elections had been rigged.

In an uncompromising address at Friday prayers, Khamenei claimed that the high turnout at the elections showed how much the Iranian people supported the regime, and blamed western powers for interfering in Iranian politics, singling out the UK as the "most treacherous".

In a thinly veiled warning to the reformist presidential challenger, Mir Hossein Mousavi, Khamenei said opposition leaders would be held responsible if they did not call for an end to the protests that have rocked Iran since last Friday's disputed election.

Significantly, an aide to Mousavi later said that he had no plans to hold a rally over the weekend, even though the opposition camp had been widely expected to mount another march on Saturday.

"Street challenge is not acceptable," said Khamenei. "This is challenging democracy after the elections."

Speaking in front of an audience of tens of thousands, including President Mahmoud Ahmadinejad, Khamenei attacked foreign powers for conspiring to deligitimise the vote, and to destroy the Iranian people's trust in their political leaders. Khamenei's description of Britain as the "most treacherous" of Iran's enemies was met by roars of "Death to the UK" from the massed ranks of Basiji militiamen assembled in the prayer hall.

In response, Iran's ambassador to London was summoned to the Foreign Office and told that the supreme leader's remarks were unacceptable . Gordon Brown later said that the EU was unanimous in condemning violence against opposition protesters: "It is ... for Iran to show the world that the repression and the brutality that we've seen in these last few days is not something that is going to be repeated."

Mousavi was conspicuous by his absence from Friday prayers at Tehran University, where Khamenei was making his first public appearance since controversially endorsing Ahmadinejad's election as president.

Khamenei praised the president as "hardworking" and dismissed the idea that the election might have been rigged.

"The Islamic republic will never manipulate votes and commit treason," he said. "The legal structure in this country does not allow vote-rigging." He said that the margin of Ahmadinejad's victory - by 11m votes over Mousavi - proved that the election could not have been fixed.

He added: "If some people have doubts and evidence it should be dealt with through legal ways – only through legal ways. I will never accept illegal innovations."

Many Iranians had been hoping that the ayatollah's speech would offer up some kind of compromise with his opponents, but he showed no inclination to bend to their demands that the election be annulled.

Some followers of Mousavi had called for a boycott of prayers at the university because of the refusal to annul the result of the poll but TV pictures showed thousands of people listening to Khamenei and occasionally chanting affirmation of his words.

The speech now creates a clear dilemma for Mousavi and his supporters: do they return to the streets in open defiance of Khamenei or drop their demands? Prior to today's speech, Mousavi had called on the opposition movement to gather in Tehran tomorrow afternoon for a rally, but many may now feel too fearful of a crackdown by the authorities.

"Arm wrestling in the street must stop," Khamenei said. "I want everyone to put an end to this. If they don't stop this … they will be held accountable for all of this."

The ayatollah insisted that it was "natural" for people to support different candidates but that the foreign media was responsible for portraying supporters of Mousavi as opposed to the Islamic revolution.

"Enemies try through various media, and some of these media belong to the Zionists … they try to make believe that there is a fight between supporters of the opposition and the Islamic establishment," said Khamenei. "They have no right to say that, that is not true."

He described the 85% turnout at the election as a "great accomplishment" and a "political earthquake" for Iran's enemies. The supreme leader said he was bringing a message for "leaders of world arrogance, the western countries".

Khamenei also contrasted Barack Obama's comments about the election unfavourably with the US president's letter offering better relations with Iran. He said Iran was "raising the flag of human rights" and criticised the US invasions of Afghanistan and Iraq. "We do not need any advice on human rights from them," he said.

In an apparent attempt to acknowledge the concerns of Mousavi's supporters he admitted that Iran was not "without corruption," and said it needed to be tackled.

He also acknowledged differences between himself and the former president Ali Akbar Hashemi Rafsanjani, who has supported Mousavi and been portrayed as a potential kingmaker, and between Ahmadinejad and Rafsanjani, but at the same time praised the chair of the assembly of experts as "close" to the revolution.

Ahmadinejad and his cabinet ministers attended the prayers, as did the parliamentary speaker, Ali Larijani, the mayor of Tehran, Mohammad Baqer Qalibaf, and senior military officers from the revolutionary guards. Many of those in the audience appeared to be government employees or members of the president's militia.

Defeated presidential candidate Mehdi Karroubi, who like Mousavi has dismissed the official election result, also stayed away from the university today.