Tehran: An opposition activist has been convicted of charges of working against the ruling system and insulting the country's supreme leader and sentenced to 20 months in jail, an opposition website reported.

Ahmad Gabel was given additional sentences of exile for three years, a ban on interviews and lectures during that time, and a fine for possessing a satellite receiver. His laptop was also confiscated.

The website, kaleme.com, said the hardline Revolutionary Court in Mashad, northeastern Iran, found Gabel guilty of insulting Supreme Leader Ayatollah Ali Khamenei.

Any criticism of Khamenei is interpreted by authorities as insulting the supreme leader.

Gabel is a prominent member of the now outlawed Islamic Iran Participation Front, the largest reformist political party. He is considered a key link between reformers and top Shiite clerics in Qom, a centre of political and religious power located about 128km south of Tehran.

He was arrested last December while driving to Qom with his family to attend the funeral of the late Grand Ayatollah Hossein Ali Montazeri, Iran's most senior dissident cleric.

Gabel, one of Montazeri's followers, was freed on bail after spending 170 days behind bars. He was re-arrested in the summer after he exposed mass killing of prisoners in Mashhad, kaleme.com said.

"The reason behind the re-arrest of this researcher is because of his exposure of secret mass executions at Vakilabad prison in Mashad and his criticism of the supreme leader," the site said.