Iran's opposition leaders appealed to top clerics in the holy city of Qom on Saturday to pressure the ruling Islamic regime to release protesters and activists who they say have been tortured following last month's disputed presidential election.

The opposition hopes that enlisting the support of the clerics will provide an effective counterweight to Supreme Leader Ayatollah Khamenei, who has dismissed claims that President Mahmoud Ahmadinejad won the June 12 election through massive vote fraud.

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Khamenei has the final say over all state matters, but the nine clerics in Qom who hold the rank of "marja' taqlid," or "source of emulation," have great spiritual influence over many Iranians.

The clerics often congratulate the winners of presidential elections, but only one has done so this time. Three others have spoken out against the violent crackdown on hundreds of thousands of supporters of opposition leader Mir Hossein Mousavi who took to the streets to dispute Ahmadinejad's victory.

Mousavi, who claims he won the election, former President Mohammad Khatami and 67 other prominent reformists sent a letter to the clerics Saturday saying authorities have held protesters and activists without charges and have used torture to extract confessions.

"We call on you, the "marja' taqlid" ... to remind the relevant authorities of the damaging consequences of employing law-evading methods and warn them about the spread of tyranny in the Islamic republic system," said the letter, a copy of which was made available to The Associated Press.

At least 20 people were killed in clashes between security forces and protesters following the election, according to police, though rights groups fear the number could be much greater.

Authorities still hold hundreds of people they arrested following the protests, including some top leaders of the reform movement, and have held many of them in secret locations.

Khamenei and his supporters have tried to depict the protesters as tools of foreign countries trying to spark a revolution to topple Iran's Islamic system.

The U.S. and other Western countries have denied the allegations, but Iran's semi-official media has reported in recent weeks that prominent opposition activists have confessed to provoking riots to topple the regime.

Iran's intelligence chief, Gholam Hossein Mohseni Ejehi, said earlier this week that some of the confessions will be broadcast on state television.

The reformists criticized the regime's actions in the letter they sent Saturday, saying "they have resorted to illegal, immoral and un-Islamic methods to obtain confessions."

"What legal, Islamic or human rights code can justify the repeated torture of those who live under the banner of Islam?" it said.

The letter said the repressive methods used to obtain confessions were reminiscent of the methods employed by Iran's former shah, who was toppled by the 1979 Islamic Revolution.

"The only way out of this situation is to release all detainees and put an end to the security state imposed after the election," it said.