Iran’s supreme leader, Ayatollah Ali Khamenei, on Friday sternly cut off any compromise over Iran’s disputed presidential election. In a lengthy sermon, he declared the election valid and warned of violence if protesters continued, as they have pledged, to fill the streets in defiance of the government.

Opposition leaders who failed to halt the protests, he said, “would be responsible for bloodshed and chaos.” The tough words seemed to dash hopes for a peaceful solution to what the opposition calls a fraudulent election last week, plunging Iran into its gravest crisis since the Islamic Revolution in 1979.

Opposition leaders called for another huge rally today, setting the stage for a possible showdown between protesters and security forces, perhaps a violent one.

Another sign of possible resistance came shortly after nightfall Friday in Tehran. Cries of “Death to the dictator!” and “Allahu akbar” — “God is great” — rang from rooftops in what’s become a nightly ritual of opposition unity.

Khamenei’s sermon, which was broadcast around Iran and the world, presents Mir Hossein Mousavi, who the opposition says was the real winner of the June 12 election, with an excruciating choice. The former prime minister must decide whether to escalate his challenge to Iran’s religious leaders and risk a bloody showdown or abandon his support for a popular uprising that his candidacy inspired.

During the tough sermon at Tehran University during Friday prayers, Khamenei hardened his stance on the election results and dismissed allegations of fraud. “Perhaps 100,000 votes, or 500,000, but how can anyone tamper with 11 million votes?” he asked.

“If the political elite ignore the law — whether they want it or not — they would be responsible for the chaos and bloodshed.”

Show by government supporters

Tens of thousands of President Mahmoud Ahmadinejad’s supporters gathered for the sermon in a clear show of force to counter the mass rallies by protesters. State-controlled television showed the streets filled with people near Tehran University, where the Friday prayer is given.

The ayatollah also offered muted criticism of pro-government militias and Ahmadinejad for their roles in the crisis.

Newspapers said Thursday that a group of members of Parliament reported that the pro-government militia known as the Basij was responsible for the attack on a student dormitory in Tehran and a housing complex called Sobhan this week. Students said that two women and three men were killed.

In his sermon, Khamenei criticized those who carried out that attack. “Have you calculated the impact of going to the dormitories in the name of the leader?” he asked. “Muscle-flexing after elections is not right. Put an end to this.”

As he concluded his sermon, Khamenei invoked the names of Shiite saints and began weeping.

Among the worshippers seen on TV at the mammoth prayer hall was Ahmadinejad and one of his three election rivals, former Revolutionary Guard commander Mohsen Rezaei.

Mousavi and the other opposition candidate, Mahdi Karroubi, weren’t shown on the state TV coverage and apparently didn’t attend Khamenei’s sermon.

A spokesman for Mousavi said Friday that the opposition leader isn’t under arrest but isn’t 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.

Makhmalbaf said he hadn’t heard from Mousavi’s camp since Khamenei’s address.

Mounting U.S. criticism

The U.S. House and Senate both approved a resolution Friday condemning “the ongoing violence” by the Iranian government and its suppression of the Internet and cell phones. It also expressed support for Iranian citizens who embrace freedom.

The House voted 405-1 for the Republican-authored resolution. The lone holdout was Rep. Ron Paul, R-Lake Jackson, who said, “I have admired President Obama’s cautious approach to the situation in Iran and I would have preferred that we in the House had acted similarly.”

President Barack Obama was asked Friday in an interview with CBS News’ Harry Smith what he made of Khamenei’s remarks.

“I’m very concerned, based on some of the tenor and tone of the statements that have been made, that the government of Iran recognize that the world is watching,” Obama said.

“And how they approach and deal with people who are, through peaceful means, trying to be heard will, I think, send a pretty clear signal to the international community about what Iran is — and is not.”