Iran's opposition yesterday called for a major new protest over the disputed presidential election, defying a demand by the nation's supreme leader to end the protests and setting the stage for a potentially violent showdown in the streets.

Defeated candidate Mir Hossein Mousavi circumvented regime efforts to block Internet access by announcing the rallies on his Facebook page in Farsi and in English yesterday. "CRUCIAL Demonstration on Saturday 16:00 in Tehran and all around the world, please spread this message around," it said.

Messages urging the public to converge today for the march through the capital also flooded Twitter, and after dark yesterday, Tehran residents took to the rooftops to cry "God is great" and "Death to the dictator" in what Iranian bloggers said were numbers exceeding previous nights.

The sermon by Ayatollah Ali Khamenei, who wields nearly absolute power under Iran's constitution, ended any doubt about his alignment with hard-line President Mahmoud Ahmadinejad. And it made clear that he was not open to finding a compromise on Mousavi's demand to annul the election based on claims of massive rigging.

"I have one vote. I have given it to Mousavi. I have one life. I will give it for freedom," one Twitter post said.

Khamenei, 69, appeared to have miscalculated if he thought he could cow the opposition with his tough speech, said Mohammad Sahimi, an Iranian American professor of chemical engineering at the University of Southern California and political analyst.

"I think he has polarized the society far more than it was because he made clear what his preference is and where he stands and who he supports," Sahimi said. The opposition is "openly defying this guy. In the short run, it may it lead to violence."

Khamenei demanded an end to the protests in an uncompromising Friday sermon. In it, he endorsed Ahmadinejad's June 12 landslide reelection, dismissed allegations that the result was fixed, and signaled that a major crackdown would be launched if his order for the protests to stop was not obeyed.

"Political-party leaders should be very careful about what they say and do," Khamenei told tens of thousands at Tehran University, a center of the antigovernment movement, as Ahmadinejad and other senior officials looked on. "They will be responsible for any bloodshed and any form of unrest."

"I will not allow any illegal initiative. If the laws are broken today, no election will be immune in the future," Khamenei said, according to an English translation aired by Press TV, the English-language version of state-run television.

In an apparent bid to discredit the swelling opposition movement, Khamenei accused the United States, European powers, and Israel of fomenting the worst political turmoil to convulse this nation of 67 million since the 1979 Islamic Revolution, which toppled the shah.

U.S. and European "agents started to cause riots in the street. They caused destruction, burned houses; theft and insecurity prevailed," Khamenei said. "These are the servants of the Westerners, Zionist agents and their servants."

Khamenei called Britain "the most evil" Western nation, prompting the British Foreign Office in London to call in the Iranian ambassador to Britain for a formal protest.

Press TV, whose reports were available online, quoted the top administrator of Tehran province as saying that no permission had been given for today's protest march.

The march - dubbed "From Revolution to Freedom" - was called by Mousavi; Mohsen Rezaei, a former Revolutionary Guard commander who officially placed third in the election; and the Association of Combatant Clerics, moderate religious scholars loyal to former Presidents Mohammed Khatami and Ali Akbar Hashemi Rafsanjani.

Mousavi had called over his Web sites for protests yesterday but postponed them until today to avoid violence after state-run media announced that a huge crowd, much of it believed to comprise Basij militia members, would hear Khamenei's sermon.

Significantly, however, he did not cancel today's demonstration after the speech.

A spokesman for Mousavi said yesterday that the opposition leader was not under arrest but was not allowed to speak to journalists or stand at a microphone at rallies. Iranian filmmaker Mohsen Makhmalbaf told the Associated Press from Paris that it was even becoming difficult to reach people close to Mousavi.

Claims that Ahmadinejad's victory was rigged ignited protests beginning in the early hours of June 13 by hundreds of thousands - perhaps millions - of Iranians in Tehran and other cities that have been punctuated by violent clashes pitting demonstrators against security forces and the Basij.

In his sermon, Khamenei said it was impossible that the 11 million-vote margin between Ahmadinejad and Mousavi was the result of rigging.

"If there was less than that, we would say there are doubts," he said, "but how can 11 million votes be replaced or changed? Don't allow enemies to destroy and ruin the celebration of our elections."

Foreign reporters who had received visas to cover the election were ordered to leave the country, and those based in Tehran have been prohibited from leaving their offices to cover the demonstrations.

However, the regime's Internet censors have been unable to prevent Iranians from posting reports, pictures, and videos of the crisis to Twitter, Facebook, and other online outlets.

This article contains information from the Associated Press.