Jailed Iranian reformists are believed to have been tortured in an attempt to force them into TV "confessions" of a foreign-led plot against the Islamic regime.

According to Iranian websites, the "confessions" are aimed at implicating Mir Hossein Mousavi and Mehdi Karroubi, the defeated reformist candidates in this month's presidential poll, 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 notorious Evin prison since being arrested in a mass round-up of opposition figures following Mahmoud Ahmadinejad's disputed re-election.

The three, who all served in the government of the former reformist president, Mohammad Khatami, are among several hundred activists, academics, journalists and students detained in a crackdown coinciding with the brutal suppression of street protesters who believe the election was stolen.

Fellow prisoners are reported to have heard screams of pain from Tajzadeh, a former deputy interior minister, and Ramezanzadeh, who was Khatami's government spokesman, during interrogations at Evin's section 209, which is reserved for political prisoners and run by the hardline intelligence ministry.

Aminzadeh, an ex-deputy foreign minister, was heard shouting "I am not going to give interviews."

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

The Iranian authorities have used this technique before to humiliate and discredit opponents. In 2007, state television aired "confessions" by US-Iranian academics Haleh Esfandiari, Kian Tajbakhsh and Ramin Jahanbegloo in which they said they had worked with pro-democracy groups that the regime claimed were plotting its downfall.

This week, state television broadcast interviews with several people admitting to being "terrorists" after purportedly taking part in street demonstrations.

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. After the president was first elected four years ago, Tajzadeh told the Guardian that Ahmadinejad's leading supporters wanted to create an atmosphere similar to that under the Taliban in Afghanistan.

Renewed fears have been voiced over the health of another jailed reformist, Saeed Hajarian, a former Khatami adviser who is severely disabled from a failed assassination attempt nine years ago.

Reports of Hajarian's death on blogs and Twitter 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.

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.