A group of opposition activists went on trial on Saturday in Tehran on charges of rioting and conspiring against the ruling system in connection with protests late last year, Iran's state media reported.

The Human Rights Watch says Iran's post-election unrest is a "full-blown human rights crisis" and calls on Tehran to free government critics detained during the crackdown.

Iran's opposition leader on Friday pledged to remain defiant in the face of new threats - including calls by hard-liners for his execution - and said he was ready to sacrifice his life in defense of the people's right to hold peaceful protests against the government.

In the bloodiest protests in months, groups of emboldened demonstrators on Sunday chanted slogans against Khamenei, casting aside a taboo on personal criticism of the leader.

Opposition activists said Iranian security forces rounded up at least seven prominent activists on Monday, stepping up a crackdown on the country's pro-reform movement a day after eight people, including the nephew of the chief opposition leader, were killed in anti-government protests.