Iran's opposition leader on Friday pledged to remain defiant in the face of new threats - including calls by hard-liners for his execution - and said he would sacrifice his life in defense of the people's right to protest peacefully against the government. Mir Hossein Mousavi's remarks come after the worst unrest since the immediate aftermath of the disputed June presidential election. At least eight people died during anti-government protests on Sunday, including Mousavi's nephew. In one of his strongest statements to date, Mousavi said he was "ready for martyrdom" - the sacrifice of one's life for a higher cause - and lashed out at the bloody crackdown the authorities are waging against the opposition. He said the government was making more mistakes by resorting to violence and killings, and that it must accept the people's rights to hold peaceful demonstrations. Iranian hard-liners have called for the execution of Mousavi and other opposition figures, while a previously unknown group claimed in an online posting that suicide squads were ready to assassinate opposition leaders should the judiciary fail to punish them within a week. Iran's state prosecutor on Thursday warned opposition leaders could be put on trial if they don't denounce this week's anti-government protests. "I explicitly and clearly state that an order to execute, murder and imprison (opposition leaders) ... won't resolve the problem," Mousavi said in a statement on his Web site, Kaleme. "I'm not afraid to be one of the martyrs people have offered in the struggle for their just demands." The confrontation between clerical rulers and their opponents returned to the streets in recent weeks, after a harsh crackdown immediately following the June 12 balloting all but crushed the opposition movement. One of those killed in clashes Sunday between security forces and opposition protesters was Mousavi's nephew, Ali Mousavi. He was gunned down but authorities claimed police didn't use firearms and said the nephew was "assassinated" by unknown assailants. The nephew was buried Wednesday in a hastily organized ceremony that was attended by the opposition leader and other family members. Authorities had taken the body from the hospital earlier in the week in what was seen as an attempt to prevent the funeral from turning into another pro-opposition protest. Hard-liners have become especially furious after some pro-opposition protesters chanted slogans against Supreme Leader Ayatollah Ali Khamenei - a taboo in Iran, where the supreme leader is considered answerable only to God. Hard-liners consider opposing Khamenei the same as opposing God. Deputy head of judiciary, Ebrahim Raisi, called pro-opposition protesters enemies of God, or mohareb in Farsi - a crime punishable by death under Iranian law. "Those who riot ... insult sanctities ... this is undoubtedly proof" of defying God, said Raisi in comments broadcast Friday on state radio. Sunday's unrest was followed by two days of pro-government protests Wednesday and Thursday in which crowds called for Mousavi's execution and that of another opposition figure, Mahdi Karroubi. Both Mousavi and Karroubi were losing candidates in the June election, in which Mahmoud Ahmadinejad was declared the winner. The opposition argues the election was rigged and that Ahmadinejad won by fraud. Some government supporters at the two days of rallies wore white funeral shrouds to symbolize a willingness to die in defense of Iran's clerical rulers. Several hundred turned out for demonstration Thursday in southern Teheran outside Khamenei's offices. In his statement Friday, Mousavi also denounced hard-liners who he said preached violence from state-funded podiums in the name of Islam, a reference to cleric Ahmad Alamolhoda who called the Khamenei's opponents "cows and goats." "Encouraging the killing of people ... is a tragedy carried out by specific individuals and the state TV," Mousavi said, adding that efforts to silence the opposition "through arrests, violence and threats," would not succeed. Iran, Mousavi said, was in a "serious crisis" but killing protesters will only make the opposition stronger. He cited the founder of Iran's 1979 Islamic revolution, late Ayatollah Ruhollah Khomeini's words: "Kill us, we will become stronger." Mousavi asked the authorities to release all detained protesters - more than 500 have been arrested since Sunday - and end the media crackdown. But speaking during Friday prayers in Teheran, hard-line cleric Ayatollah Ahmad Jannati said those arrested should be kept in jail. He said the protesters were "corrupt on earth" - an accusation that also carries the death sentence in Iran.