A defeated candidate in Iran's disputed presidential election alleged Thursday that some opposition protesters have been tortured to death in detention.

Iranian presidential candidate Mehdi Karroubi joins a crowd of supporters in Tehran in June. (Demotix/Reuters)In a statement on his party's website, former parliamentary Speaker Mehdi Karroubi said demonstrators have been beaten to death "just for chanting slogans."

He called for the formation of an independent panel to review his evidence in "a calm atmosphere," the website said.

The allegations come several days after Karroubi said several male and female detainees at the Kahrizak prison were raped by their jailers, a charge vehemently denied as a "lie" on Wednesday by current Iranian parliamentary Speaker Ali Larijani.

Meanwhile, an independent investigator appointed by the UN Human Rights Council in Geneva said Thursday he has received credible claims that Iranian protesters detained after the June 12 presidential elections were abused in prison.

"I truly believe that these complaints will stand up to scrutiny," Manfred Nowak told The Associated Press.

'Deep change' needed: Moussavi

Reformists have seized on outrage over the mistreatment of detainees at Kahrizak prison as a way to keep pressure on President Mahmoud Ahmadinejad, who they claim stole the election through massive fraud.

In a statement, opposition leader Mir Hossein Moussavi said the abuses at the prison following the election show the need for "deep change" in the country.

Iran has confirmed at least 30 people have died in the worst internal unrest since the 1979 Islamic Revolution, though human rights groups believe the death toll is probably far higher.

The Iranian regime has acknowledged about 4,000 people were detained in a crackdown on massive demonstrations in the weeks following the election, but insists most have been released.

Tehran has barred foreign media organizations from gathering information inside the country, making reports of violence and mistreatment extremely difficult to verify.

Ahmadinejad's election victory has been affirmed by Iran's Supreme Leader, Ayatollah Ali Khamenei, who has claimed the demonstrations are part of a foreign conspiracy to stir up unrest in the country.

With files from The Associated Press