In his speech at a mosque at Tehran University, Khamenei spoke for nearly two hours and insisted there was no fraud in Iran's disputed presidential elections and said that the state would not cheat. "The people have made their choice," he said.
He also gave Ahmadinejad his full support. "It's natural to have differences of opinion," he said. "But I believe the president's [views] are closer to what they should be."
Friday prayers at the mosque at Tehran University -- a huge, outdoor, open-air arena -- are the premier forum for Iran's religious leaders. The location is full of historical meaning. It was on the campus of Tehran University that the Islamic revolution was born in 1979. And today, it is certain much of Iran would have been watching and listening closely to Khamenei's words.
Khamenei blamed "Iran's enemies" in the West, singling out the United States, Great Britain and Israel for undermining the elections. The "Zionist-owned Western media," he said, are to blame too.
"The enemies [of Iran] are targeting the Islamic establishment's legitimacy by questioning the election and its authenticity before and after [the vote]," he said.
"After street protests, some foreign powers ... started to interfere in Iran's state matters by questioning the result of the vote. They do not know the Iranian nation. I strongly condemn such interference," Khamenei said.
Turning his attention to the United States, the ayatollah said, "American officials' remarks about human rights and limitations on people are not acceptable because they have no idea about human rights after what they have done in Afghanistan and Iran and other parts of the world. We do not need advice over human rights from them."
More ominously for the opposition, Khamenei made it clear he disapproved of any "illegal" challenges.
"Elections are decided by the ballot box not in the streets," he said.
Equally ominously, he explicitly warned opposition leaders, "If rallies continue, they will be responsible for violence." The opposition, so far, is undeterred and have not canceled plans for a rally on Saturday.
For the first day since Monday, there were no reports of opposition protests in Tehran, but according to the Associated Press, residents of Tehran continued to climb to their rooftops to chant "Death to the Dictator!" in open defiance of Iran's supreme leader's pleadings today.
BBC reporter Jon Leyne, who has covered Iran for several years, described the atmosphere on the streets of Tehran as very tense.
"Government supporters have come out in force," he said on "Good Morning America." "Great hostility to the Western media, and to people they believe are trying to undermine this country."
Leyne said Khamenei's tough rhetoric against the West and warnings to Iranian protesters that they face dire consequences if they don't stop rallies does not bode well for the country.
"I think this is a black day for Iran and its relations with the outside world," he said. "A real, new phase in this crisis from the supreme leader."
In Paris, Mousavi's unofficial spokesman, Iranian filmmaker Mohsen Makhmalbaf, told ABC News that the Mousavi camp is asking other countries to not recognize Ahmadinejad's election.