TEHRAN (Reuters) - Iranian authorities have tightened pressure on their opponents by staging what former president Mohammad Khatami condemned on Sunday as a "show trial" of 100 reformists accused of trying to instigate a "velvet revolution."
The trial was the latest shot in an official campaign to snuff out defiance by those who say Iran's June 12 election was rigged to ensure the re-election of President Mahmoud Ahmadinejad, due to be sworn in by parliament on Wednesday.
Supreme Leader Ayatollah Ali Khamenei, who has endorsed the election result and demanded an end to protests, will formally approve the hard-line incumbent for a second term on Monday.
Khatami, several of whose close associates were in the dock on Saturday, said the trial violated Iran's constitution.
"Such show trials will directly harm the system and further damage public trust," he said on his website (www.khatami.ir). Another court session is scheduled for Thursday.
Defeated election candidate Mirhossein Mousavi dismissed what he said were confessions made under duress.
"The torturers and interrogators have gone to such lengths that their victims are among those who gave great services to Iran in the past," he said on his website Ghalamnews.
"Soon we will see the trials of those who committed these crimes, the torturers and interrogators."
Iranian officials deny any fraud in the election, in which Ahmadinejad was declared to have won 63 percent of 40 million votes cast, against 34 percent for his nearest rival Mousavi -- who says the next government will be illegitimate.
Even some hardliners criticized the trial and the official portrayal of protesters as bent on overthrowing the system.
Emad Afrough, a former pro-Ahmadinejad lawmaker, was quoted by Etemad daily as saying that people who described election protests as a velvet revolution should themselves be tried.
The mass trial of dozens of reformists, including senior officials such as Khatami's former vice-president, Mohammad Ali Abtahi, paraded in prison dress without his clerical turban, has no precedent in revolutionary Iran's 30-year history.
Proceedings were closed to all but state media. Many of the defendants had spent weeks in jail without access to lawyers. Some, like Abtahi, appeared to have lost weight and spirit as they assured the court that the election was free and fair.
The defendants were charged with rioting, attacking military and government buildings, having links with armed opposition groups and conspiring against the ruling system, the official IRNA news agency said. Some admitted guilt.
State television showed Abtahi testifying that the vote was valid and apologizing for his "misjudgments." He said Mousavi, Khatami and former President Akbar Hashemi Rafsanjani had taken an oath of mutual support before the vote, IRNA reported.
This was denied by Rafsanjani, an influential cleric and veteran of the 1979 revolution who heads the Assembly of Experts that appoints and can, in theory, dismiss the supreme leader.
Although Revolutionary Guards and Basij militia suppressed huge post-election rallies, opposition leaders remain defiant.
Their supporters again braved batons and tear gas last week to mark the 40th day after the death of Neda Agha-Soltan, a young woman shot on the sidelines of a protest. Film of her last moments was broadcast on the Internet.
The aftermath of the election has exposed deep schisms within Iran's clerical and political elite, with Ahmadinejad coming under fire from many conservatives as well as reformists.
His appointment as vice-president of a man mistrusted by hardliners for remarks on Israel and for hosting an event they deemed un-Islamic prompted a veto from Khamenei last month.
Ahmadinejad veered close to defying the supreme leader by delaying a week before obeying his order and then naming the same man, Esfendiar Rahim-Mashaie, as his chief of staff.
He also sacked a hard-line intelligence minister who had criticized his actions, while his culture minister resigned.
Ahmadinejad told rivals on Friday that trying to split him from Khamenei was futile because they were like father and son.
Yet the same day, hard-line cleric Ayatollah Ahmad Jannati publicly rebuked Ahmadinejad in a nationally broadcast sermon.
And on Sunday, the president's media adviser and close ally, Ali Akbar Javanfekr, made public his resignation, which local media said had been offered two weeks ago but not accepted.
After his inauguration, Ahmadinejad has two weeks to submit his cabinet list to the mostly conservative parliament, which may resist if he only names members of his inner circle.
One prominent conservative MP, Ali Motahari, criticized the president's handling of detainees, asking why he had waited until the supreme leader had ordered a "sub-standard" detention center at Kahrizak to close before intervening himself.
"He could have acted sooner and he could have treated others, including detainees, more kindly," Motahari told the semi-official Mehr news agency on Sunday.
"But unfortunately, this did not happen and some detainees, including Mohsen Ruholamini, were treated violently by some people," he added, alluding to the death in jail of the son of a senior aide to conservative presidential hopeful Mohsen Rezaie.
The political uncertainty has posed fresh challenges for Western powers which had hoped to engage the Islamic Republic in substantive talks on its nuclear program, which they suspect has military purposes, not only civilian ones as Iran insists.
Another potential source of friction with the United States arose on Saturday when Iran arrested three American hikers who an Iraqi Kurdish official said had strayed across the border.
Writing by Alistair Lyon and Parisa Hafezi; editing by Andrew Roche