Iran's defeated presidential candidate Mehdi Karoubi said on Thursday that some detainees taking part in post-election protests have been tortured to death.

We observe that in an Islamic country some young people are beaten to death just for chanting slogans in (the post- election) protests, Karoubi said in his website.

Some of the detainees said they were forced to take off their clothes. Then they were made to go on their hands and knees and were ridden (by prison guards), Karoubi said on thursday.

Or the prison authorities put them on top of each other while they were naked, he added.

He also said on Sunday that some protesters, both men and women, had been raped in detention. Iran authorities rejected his allegation as baseless.

Many of the post-election detainees were held in south Tehran's Kahrizak prison, built to house people breaching vice laws.

At least three people died in custody there and widespread anger erupted as reports of abuse in the jail spread.

Iran has charged dozens of people with spying and aiding a Western plot to overthrow its system of clerical rule following June's presidential election.