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 3 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 an official Islamic Republic News Agency report.

Meanwhile, officials announced plans to set up a special court and warned that anyone who encouraged more demonstrations, including opposition figurehead Mir Hossein Mousavi, would be 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 ... and to clean the country of these plotters and hooligans," said the statement, according to the agency.

Despite the warnings, Mr. Mousavi, who the government has announced suffered a landslide defeat to President Mahmoud Ahmadinejad, called upon 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 and be like kind, brokenhearted parents to poorly behaving children in the law enforcement forces."

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

But the Guardian Council, whose members are appointed directly or indirectly by Ayatollah 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 council spokesman Abbas-Ali Kadkhodai, according to the Khabar Online news Web site.

Chatham House, a British think tank, published a study over the weekend in which it found irregularities by comparing Iranian presidential voting in 2009 and 2005 against the 2006 census published by the official Statistical Center of Iran. The report found that two conservative provinces reported turnouts of more than 100 percent, and that in one-third of all provinces, this year's official results would have meant that Mr. Ahmadinejad won not only all the conservative voters from 2005 but also the centrist voters from then and all new voters -- as well as 44 percent of reformist voters.

Iran continued to reel from days of protests that culminated in chaotic fighting Saturday between security forces and demonstrators.

Authorities said 13 people were killed and 20 injured Saturday. Iranian officials say the casualties were caused by "armed terrorists" who infiltrated the rallies, burned down a mosque and two gas stations and attacked a military base, according to a report on the Web site of the state-owned Press TV news channel.