Mehdi Karroubi and other opposition figures have faced increasing pressure
Leading Iranian opposition figure Mehdi Karroubi has said he remains defiant despite threats, and that he and his family are "prepared for any disaster".
He also accused Tehran of suppressing people in the name of religion.
Mr Karroubi, who stood in June's disputed presidential elections, also said he survived an attack last week when a car he was in was fired on.
Meanwhile, Iran's top prosecutor called for "stong action" against those behind the "post-election sedition".
Gholam Hossein Mohsen Ejeie said calls by some hardline clerics to put opposition leaders on trial should be answered, Iran's Isna news agency reported.
President Mahmoud Ahmadinejad claimed victory in the vote, but opposition leaders said the election was rigged.
The election led to a series of street protests and crackdowns.
In a letter on his party's website, Mr Karroubi said that he remained "firm in the path that I have chosen".
"I have not faltered on my way in the face of increasing intimidation and threats," he said.
"I have become more determined, and prepared myself and my children for any disaster."
The Iranian protest movement has developed into the biggest challenge to the government since the Islamic Revolution in 1979.
Thousands of people have been arrested and dozens killed.
Mr Karroubi and others have faced increasing pressure from the authorities, with some hardliners labelling them as "mohareb" - enemies of God who can be sentenced to death under Iran's Sharia law.
"May God stand witness how they have turned a political dispute into a religious war to suppress people demanding political rights in the name of religion," Mr Karroubi said.
"We were not aware that reclaiming your vote and protesting against political robbery is equal to being corrupt on earth and mohareb in these gentlemen's eyes."
Mr Karroubi said last week that his armoured car had been fired on in the city of Qazvin, where he had been attending a mourning ceremony for opposition supporters killed in protests.
Mr Karroubi has become one of the leading figures of the opposition reformist movement, along with another defeated candidate, Mir Hossein Mousavi.
Mr Mousavi said earlier this month that he was not afraid to die for the cause of reform.
A nephew of Mr Mousavi was among anti-government protesters killed during a crackdown by authorities in late December.