Opposition leader Mirhossein Mousavi warned today that Iran was in "serious crisis" and called for the immediate release of supporters arrested after the June presidential vote.
In a statement published on his Kaleme website, Mr Mousavi said arresting him or fellow opposition leader Mehdi Karoubi would not calm the situation.
"I am not afraid to die for people's demands ... Iran is in serious crisis ... Harsh remarks ... will create internal uprising ... the election law should be changed ... political prisoners should be freed," his statement said.
Anti-government protests began in Iran after its disputed June 12th presidential election, which secured President Mahmoud Ahmadinejad's re-election.
The continuing protests have plunged Iran into its worst internal crisis in the Islamic Republic's 30-year history. Opposition leaders say the presidential vote was rigged. The government denies this.
The authorities have intensified their crackdown on the opposition since Sunday, when eight people - including a nephew of Mr Mousavi - were killed in protests on the day of the Shia Muslim ritual of Ashura.
The government has accused opposition leaders of fomenting unrest and called for them to be punished.
A representative of Supreme Leader Ayatollah Ali Khamenei said earlier this week that opposition leaders were "enemies of God" who should be executed under the country's sharia, Islamic law.
Authorities have arrested at least 20 pro-reform figures, including three senior advisers to Mr Mousavi, his brother-in-law and a sister of Iranian Nobel Peace Prize winner Shirin Ebadi.
Iran's police chief has warned Mr Mousavi's supporters they will face harsh treatment unless they halt their "illegal" rallies.