TEHRAN, Iran - Government authorities stepped up their crackdown on protesters yesterday, as Iranian officials for the first time acknowledged evidence of voting irregularities in this month's presidential election, the issue that has sparked the largest street demonstrations since the Islamic Republic was established three decades ago.

An initial probe into the June 12 presidential election has shown that the number of ballots cast exceeded the number of registered voters in 50 locales, a discrepancy affecting three million votes or more, according to the spokesman for the Guardian Council, a body of jurists and clerics in charge of safeguarding the country's constitution.

The council will deliver its final verdict on the disputed election by tomorrow, according to a report by the official Islamic Republic News Agency.

Meanwhile, officials announced plans to set up a special court and warned that anyone who encouraged more demonstrations - including opposition figurehead Mir Hossein Mousavi - was subject to arrest.

In some of its sternest remarks yet, the Revolutionary Guard announced that anyone who continued to confront the security forces "will be considered a threat" to the system, the news agency reported.

"The guardians of the Islamic Revolution and the courageous Basiji," a pro-government militia, "are determined to act strongly to return peace and tranquillity to society," the statement said.

Despite the warnings, Mousavi, who the government announced suffered a landslide defeat to Mahmoud Ahmadinejad, called on his supporters yesterday to gather their strength and continue peaceful protests, sharpening his conflict with the government.

"The protest against vote-rigging and untruth is your right," he said in a statement carried on a news Web site affiliated with his presidential campaign. "In your protest keep avoiding violence."

Iran's supreme leader, Ayatollah Ali Khamenei, has ordered the protesters to halt their marches and on Friday ridiculed the vote-fraud allegations as he stood strongly behind Ahmadinejad in his prayer sermon.

But the Guardian Council, whose members are appointed directly or indirectly by Khamenei, indicated that the vote count was indeed problematic.

"It has yet to be determined whether the possible change in the tally is decisive in the election results," said Abbas-Ali Kadkhodai, a spokesman for the council.

The street combat came after Khamenei ordered demonstrators off the streets in a Friday prayer sermon interpreted as a call to semiofficial pro-government vigilantes to crack down on the rallies.

Iranian authorities have blamed the West for stirring up the unrest over the vote-counting. In public statements, they have targeted Britain, which launched the popular BBC Persian-language news channel this year.

Following threats and the expulsion of the BBC Tehran bureau chief, the British Embassy ordered the families of its expatriate staff out of the country yesterday.

But the eruption of violence and popular discontent over the election also illustrates a rift within the highest echelons of the country's clerical leadership between Khamenei and Ayatollah Ali Akbar Hashemi Rafsanjani, the head of a council that oversees the supreme leader.

Politicians and clergy have been huddling for days in discussions over ways to resolve the crisis, which has proved as divisive in the corridors of power as on the street.

Yesterday, heavy contingents of antiriot police safeguarded key downtown squares, including Haft-e-Tir, where police using tear gas chased off a group of about 200 demonstrators paying tribute to a young woman whose violent shooting death Saturday was captured on video and circulated around the world.

Neda Agha-Soltan, 26, whose first name means "the calling" in Farsi, was killed by a gunshot to the heart, and her agonizing death in the street has turned her into an icon for Iranians.

Black-clad antiriot police could be seen dragging off two young men who were distributing political tracts downtown. Basiji militiamen could be seen beating and carting off a young woman who refused to allow them to inspect her handbag.

Ebrahim Raisi, a top official in Iran's judiciary branch, said tribunals would be set up to process hundreds of "rioters" caught in security sweeps during the unrest. In a pointed warning to protest leaders, he added that "any comment, any writing, or any move that might provoke or encourage people to create insecurity will be considered crimes."

The Tehran prosecutor's office said it had arrested at least 457 people in Saturday's unrest, but a source inside Evin prison said nearly 1,000 had been detained.