Ayatollah Ali Khamenei sided with hardline President Mahmoud Ahmadinejad and offered no concessions to the opposition. He effectively closed any chance for a new vote by calling the June 12 election an "absolute victory."
The speech created a stark choice for candidate Mir Hossein Mousavi and his supporters: Drop their demands for a new vote or take to the streets again in blatant defiance of the man endowed with virtually limitless powers under Iran's constitution.
Pro-Mousavi Web sites had no immediate reaction to Khamenei's warning and no announcement of any changes in a protest planned for 4 p.m. Saturday.
Khamenei accused foreign media and Western countries of trying to create a political rift and stir up chaos in Iran. Iranian leaders often blame foreign "enemies" for plots against the country, but Khamenei's comments suggest Iran could remain cool to expanding dialogue with the West and the offer of opening talks with Washington.
"Some of our enemies in different parts of the world intended to depict this absolute victory, this definitive victory, as a doubtful victory," he said. "It is your victory. They cannot manipulate it."
Khamenei said the 11 million votes that separated Ahmadinejad from his top opponent, Mousavi, were proof that fraud did not occur. Ahmadinejad watched the sermon from the front row. State television did not show Mousavi in attendance.
"If the difference was 100,000 or 500,000 or 1 million, well, one may say fraud could have happened. But how can one rig 11 million votes?" Khamenei asked during Friday prayers at Tehran University.
Mousavi and his supporters have staged massive street rallies in recent days that have posed the greatest challenge to the Iran's Islamic ruling system since the 1979 Islamic Revolution that brought it to power.
So far, the government has not stopped the protests with force despite an official ban on them. But Khamenei opened the door for harsher measures.
"It must be determined at the ballot box what the people want and what they don't want, not in the streets," he said. "I call on all to put an end to this method. ... If they don't, they will be held responsible for the chaos and the consequences."
Khamenei blamed the U.S., Great Britain and what he called Iran's other enemies for fomenting unrest. He said Iran would not see a second revolution like those that transformed the countries of the former Soviet Union.
British Prime Minister Gordon Brown said in Brussels that the European Union unanimously condemned violence against protesters in Iran, and the British Foreign Office said it had summoned Iran's ambassador to London to explain Khamenei's comments.
He remained staunch in his defense of Ahmadinejad, saying his views were closer to the president's than to those of former President Hashemi Rafsanjani, a powerful patron of Mousavi.
Khamenei's address was his first since hundreds of thousands of Mousavi supporters flooded the streets in Tehran and elsewhere in the country in rallies evoking the revolution that ended Iran's U.S.-backed monarchy. On Thursday, supporters dressed in black and green flooded downtown Tehran in a somber, candlelit show of mourning for those who have been killed in clashes since Friday's vote.
Khamenei said the street protests would not have any impact.
"Some may imagine that street action will create political leverage against the system and force the authorities to give in to threats. No, this is wrong," he said.
The supreme leader left open a small window for a legal challenge to the vote. He reiterated that he has ordered the Guardian Council, an unelected body of 12 clerics and Islamic law experts close to the supreme leader, to investigate voter fraud claims.
The Council has said it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities.
He stressed that the four candidates were part of the country's Islamic system and reminded listeners that Mousavi was prime minister of Iran when Khamenei was president in the 1980s.
"All of them belong to the system. It was a competition within the ruling system," he said.
So far, protesters have focused on the results of the balloting rather than challenging the Islamic system of government. But a shift in anger toward Iran's non-elected theocracy could result in a showdown over the foundation of Iran's system of rule.
Ahmadinejad has appeared to take the growing opposition more seriously in recent days, backtracking Thursday on his dismissal of the protesters as "dust" and sore losers.
The crowds in Tehran and elsewhere have been able to organize despite a government clampdown on the Internet and cell phones. The government has blocked certain Web sites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence.
Text messaging, which is a primary source of spreading information in Tehran, has not been working since last week, and cell phone service in Tehran is frequently down. The government also has barred foreign news organizations from reporting on Tehran's streets.
The BBC said it was employing two new satellites to help circumvent Iranian jamming of its Persian-language service.