Iranian authorities have tightened pressure on their opponents by staging what former President Mohammad Khatami derided 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 hardline 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).

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, Mirhossein Mousavi -- who says the next government will be illegitimate.

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 apologising 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.

Khatami dismissed what he said were confessions made under duress, adding: "The detainees were not informed in advance about the court session and lacked access to lawyers."    

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 when they marked the 40th day after the death of Neda Agha-Soltan, a young woman shot dead on the sidelines of a protest. Film of her last moments was broadcast on the Internet.

The aftermath of the election has also exposed deep schisms within Iran's clerical and political elite, with Ahmadinejad coming under fire from many conservatives and even from his hardline allies, as well as from moderates and 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 his hardline intelligence minister, who had criticised his actions, and his culture minister resigned.

The president warned rivals on Friday that their efforts to drive a wedge between him and Khamenei were futile because their relationship was akin to that between "father and son".

Nevertheless, hardliners such as Ayatollah Ahmad Jannati, the head of the Gua