Iran's crackdown on opposition protests following June's disputed presidential election was a "human rights disaster," U.S.-based Human Rights Watch said.
The rights group also said in a report that Iran has staged hundreds of show trials of detained opposition protesters.
Iran has dismissed previous criticisms of its human rights record. It has said that the opposition protests were illegal and have been orchestrated by foreign powers including the United States and Britain to undermine the Islamic Republic.
Iran witnessed its worst internal strife since the Islamic revolution in 1979 when supporters of opposition candidates who lost to hardline President Mahmud Ahmadinejad took to the streets, leading to violent clashes with security forces.
Thousands were detained. Most have been freed but more than 80 were jailed for up to 15 years and five were sentenced to death.
The Human Rights Watch report said the post-election crackdown had turned into "a human rights disaster."
"The Iranian judiciary's show trials of hundreds of demonstrators and dissidents ranks among the most absurd displays of prosecutorial abuse I have witnessed in recent memory," HRW's Middle East Director Joe Stork said at a news conference in Dubai to announce its annual report.
The Human Rights Watch report said many of the detainees had been coerced to confess to vaguely-worded crimes during the trials. Researcher Faraq Sanei said Human Rights Watch had documented 26 such cases of torture or coerced confessions.
Human Rights Watch is shunned by the government and Stork said repeated visa requests had been unsuccessful.
The report said the Iranian government had also targeted the media since the election as well as workers demanding rights.
"Since 2006, authorities have responded harshly to workers, teachers, and women's rights groups who advocate for better working conditions, better wages, benefits, and demands for changes in discriminatory laws," it said.
"In 2009 the authorities arrested union leaders, women activists, and suppressed gatherings of teachers and workers."
Iran has banned Iranians from contacting 60 organisations including the BBC, Human Rights Watch and U.S.-funded broadcasters that Tehran says are suspected of being involved in Western efforts to topple the clerical establishment.
The authorities have signalled they will tolerate no more protests after eight people, including a nephew of opposition leader Mir Hossein Musavi, were killed in fiery demonstrations in December during the Shi'ite ritual of Ashura.