TEHRAN — In his first public response to days of mass protests, Iran’s supreme leader, Ayatollah Ali Khamenei, sternly warned opposition supporters on Friday to stay off the streets and raised the prospect of violence if the defiant, vast demonstrations continued.
Opposition leaders, he said, will be “responsible for bloodshed and chaos” if they do not stop further rallies.
He said he would never give in to “illegal pressures” and denied their accusations that last week’s presidential election was rigged, praising the officially declared landslide for the incumbent, Mahmoud Ahmadinejad, as an “epic moment that became a historic moment.”
He spoke somberly for more than an hour and a half at Friday Prayer to tens of thousands of people at Tehran University, with Mr. Ahmadinejad in attendance. His sermon was broadcast over loudspeakers to throngs in the adjoining streets, and the crowds erupted repeatedly in roars of support. Opposition supporters had spread the word among themselves not to attend.
“Street challenge is not acceptable,” Ayatollah Khamenei said, according to a rendering by the BBC. “This questions the principles of election and democracy.”
There was no immediate response from opposition leaders.
The ominous speech sharply increased the confrontation between Iran’s rulers and supporters of the main opposition candidate, Mir Hussein Moussavi, who have accused the authorities of rigging the vote and called for or encouraged the huge silent marches in Tehran for the last four days. No rally was planned for Friday, and opposition supporters did not appear to be gathering impromptu.
But on Saturday, a group of reformist clerics loyal to the former President Mohammed Khatami planned to demonstrate against the election results, saying they had been given rare official permission. Some news reports, however, said that the gathering had been banned.
Ayatollah Khamenei instructed dissenters to pursue their complaints about the June 12 ballot through legal channels, insisting that the turnout — officially put at 85 percent — showed it to be a reflection of the national will.
Reiterating his Saturday affirmation of the official election results, he said that the participation, as officially reported, had shown “the hand of the Lord of ages supporting such a great development.”
“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,” Ayatollah Khamenei said, framing his position as a commitment to the law and the orderly functioning of government.
“If we break the law, we will have to do it in every election and no election would be immune,” he said. “This is wrong. This is the beginning of dictatorship.”
He said that the margin of victory — 11 million votes — accorded to Mr. Ahmadinejad in the official tally was so big that it could not have been falsified.
“How can 11 million votes be replaced or changed?” he said. “The Islamic Republic would not cheat and would not betray the vote of the people.”
Some Iranians, who spoke in return for anonymity for fear of official reprisals, said the sermon showed that Iran was in the grips of what one person called “an all-or-nothing showdown” between the authorities and reformists.
Iranians had been looking to the ayatollah’s appearance for clues as to whether the authorities were prepared to bend to opposition demands. But he showed no readiness to countenance their demands that the election be annulled or to veer from the line he has taken since he endorsed the vote almost as soon as the results were made known last Saturday.
Ayatollah Khamenei blamed “media belonging to Zionists, evil media” for seeking to show divisions between those who supported the Iranian state and those who did not, while, in fact, the election had shown Iranians to be united in their commitment to the Islamic revolutionary state.
“There are 40 million votes for the revolution, not just 24 million for the chosen president,” he said, referring to the official count that gave Mr. Ahmadinejad more than 60 percent of the ballot.
He said the election “ was a competition among people who believe in the state.”
He also spoke of the religious roots of “our revolutionary society.”
“Despite all the diversions, our people are faithful,” he said, but urged young Iranians to lead more spiritual lives. “The youth are confused. Being away from spirituality has caused confusion. They don’t know what to do.”