Sahamnews.org said Karroubi made the call in a letter to Iran's judiciary chief, Ayatollah Sadeq Larijani, that was posted on the website as he was under "complete house arrest" and unable to deliver it.

"I have a request to you that as soon as possible and through legal channels, a public court be set up so the indictment and defence are heard," Karroubi said in the letter, according to Sahamnews.org.

"It should be a fair and public court so the people get to know the truth," he said, stressing that his "firmness in defending the legitimate right of the people was unshakeable."

The letter was posted on the website on the same day that a group supporting Karroubi and fellow opposition leader Mir Hossein Mousavi have called for nationwide anti-government rallies.

The call from the Council for Coordination of the Green Movement Hope was posted on Sahamnews.org and Mousavi's website, Kaleme.com, on Thursday, prompting a warning from authorities for protesters to stay off the streets.

The Sunday rallies were to commemorate two deaths at protests last Monday when security forces clashed with opposition supporters, the group said.

Karroubi and Mousavi, once pillars of the Islamic regime, have rebelled against the establishment since their rival, President Mahmoud Ahmadinejad, was re-elected in 2009.