Jailed Iranian reformists have been tortured in an attempt to force them into TV "confessions" of a foreign-led plot against the Islamic regime, it was allegedtoday, as the country's guardian council buried hopes for any significant revision of the disputed presidential election.

According to Iranian opposition websites, the "confessions" are aimed at implicating Mir Hossein Mousavi and Mehdi Karroubi, the defeated reformist candidates, in an alleged conspiracy.

Mostafa Tajzadeh, Abdollah Ramezanzadeh and Mohsen Aminzadeh, all Mousavi supporters, are reported to have undergone "intensive interrogation" sessions in Tehran's Evin prison since Mahmoud Ahmadinejad's disputed re-election.

They are among several hundred activists, academics, journalists and students detained in a crackdown coinciding with the brutal suppression of street protests.

Prisoners reportedly heard screams from Tajzadeh and Ramezanzadeh in Evin's section 209, which is reserved for political prisoners and is run by the hardline intelligence ministry. Aminzadeh, a former deputy foreign minister, was heard shouting: "I am not going to give interviews."

Amnesty International said the reports came from "very credible sources".

The guardian Council has declared there were no major violations in the vote, which it described as the "healthiest" since the 1979 Islamic Revolution. It had already rejected a call by Mousavi, for the election to be annulled because of suspected vote rigging.

a senior cleric, Ayatollah Ahmad Khatami, told worshippers at Tehran University: "I want the judiciary to ... punish leading rioters firmly and without showing any mercy to teach everyone a lesson." Ominously, he used the term "mohareb" – "one who wages war against God" – a crime punishable by death.

Khatami's sermon, broadcast live on state TV, included accusations that the unrest was supported by Israel and the US, and that foreign journalists had reported falsely. He claimed Neda Agha Soltan, who became a symbol of the opposition when her death was caught on video, was a victim of protesters, not the security forces. "Government forces do not shoot at a lady standing in a side street," he said.

Protests have continued for nearly two weeks, but faded in recent days after a crackdown by security forces and a perceived lack of leadership by Mousavi. About 20 people have been killed and hundreds beaten and arrested.

This week, state television broadcast interviews with several people admitting to being "terrorists" after purportedly taking part in street demonstrations. But forced "confessions" have been used before to humiliate and discredit opponents of the regime.

Tajzadeh's wife, Fakhrosadat Mohtashamipour, told the pro-reform website Emruz that she and a lawyer had been denied access to him since his arrest the day after the 12 June election. "Any quote or remarks made by these people in the current situation has no credibility. My husband's only crime is his efforts to secure a high turnout," she said.

Tajzadeh, 53, a member of the pro-reform Islamic Participation Front and the Islamic Revolution Mojahedin Organisation, has been a staunch critic of Ahmadinejad. Renewed fears have been voiced over the health of another jailed reformist, Saeed Hajarian, a former adviser to the reformist president Mohammed Khatami who is severely disabled from a failed assassination attempt nine years ago. Reports of Hajarian's death were dismissed by the reformist website Parlemannews, which quoted "informed sources" as saying he was in "relative health" and being given essential medication and care.

David Miliband, the foreign secretary, said at a G8 meeting in Trieste: "It is for the Iranian people to choose their government, but it is for the Iranian government to protect their people. The violence we have seen ...and the killings and the beatings are deplorable and they show a failure to protect their own people. There is a crisis of credibility not between Iran and the west, but between the Iranian counting of the votes and the Iranian people."

Meanwhile, state TV reported that the head of Mousavi's information committee, Abolfazl Fateh, had been prevented from leaving Iran for Britain, where he is a PhD student. Fars, a pro-Ahmadinejad news agency close to the country's Revolutionary Guard, said he had been banned from travelling to allow the authorities to investigate "recent gatherings", a reference to the pro-Mousavi demonstrations.