Mir Hossein Mousavi (L) pictured with his late nephew
Iranian opposition leader Mir Hossein Mousavi has said he is not afraid to die for the cause of reform.
It was his first comment since the death of his nephew and other anti-government protesters late last month.
In a statement posted on his website, Mr Mousavi also set out a five-stage solution to the crisis sparked by June's disputed elections.
He said the government, parliament and judiciary must accept "direct responsibility" for the situation.
"I am not unwilling to become a martyr like those who made that sacrifice after the election for their rightful national and religious demands," he said in the statement on his Kaleme.org website.
And he rejected the strong criticisms by Iranian hardliners who have accused opposition leaders such as himself of igniting unrest.
"Such harsh remarks... will create internal uprising... my blood is not redder than that of other martyrs," he said.
'Silence is a sin'
He called on the government to create a "transparent law" for trustworthy elections, release political prisoners, recognise press freedoms as well as the right of people to demonstrate.
"I say openly that until there is an acknowledgement of the existence of a serious crisis in the country, there will be no possibility of resolving the problems and issues."
He went on to say that the arrest or killing of either himself or other opposition leaders would "not calm the situation".
This is one of Mir Hossein Mousavi's strongest statements in recent months, says the BBC's diplomatic correspondent Bridget Kendall.
It is a defiant and very public response to recent threats from Iran's clerical leaders, some of whom this week demanded the execution of both Mr Mousavi and other opposition figures for stoking dissent against the authorities, our correspondent says.
But at Friday prayers in Teheran, Ayatollah Ahmad Jannati - who is a staunch supporter of President Ahmadinejad - said the protesters were in league with Iran's enemies, and he attacked officials who failed to denounce them.
"Some individuals who remained silent should know that, irrespective of their post, their silence is a sin... It is really treason to the Koran," he said.
Despite the rhetoric, the fourth candidate in June's presidential election - former Revolutionary Guards chief Mohsen Rezai - welcomed Mr Mousavi's comments as a basis for compromise.
He said Mr Mousavi's "retreat from rejection" of President Ahmadinejad's government, and his call for parliament and the judiciary to hold the government accountable, could be "a new start".
Mr Mousavi's nephew, Seyed Ali Mousavi, was one of at least eight people who were killed in a brutal crackdown on anti-government protests on 27 December, on the day of the Shia festival of Ashura.
He was shot in the back as security forces fired on demonstrators in Tehran, his uncle's website said. Officials have denied he was shot by the police.
RECENT UNREST IN IRAN
19 Dec: Influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri dies aged 87
21 Dec: Tens of thousands attend his funeral in Qom; reports of clashes between opposition supporters and security forces
22 Dec: Further confrontations reported in Qom
23 Dec: More clashes reported in city of Isfahan as memorial is held
24 Dec: Iran reportedly bans further memorial services for Montazeri except in his birthplace and Qom
26 Dec: Clashes reported in central and northern Tehran
27 Dec: At least eight dead following anti-government protests in Tehran; 300 reported arrested
The bodies of the protesters were "retained" for forensic tests, state media reported at the time, which prevented the rapid burials that are usual under Islamic tradition.
The Mousavi family said Seyed Ali's body had been taken from the hospital without their permission, and accused the government of removing it to prevent his funeral becoming a rallying point for more protests.
The body was returned later in the week on condition the family held a quiet funeral, which they complied with, opposition websites reported.
Sunday's violence was the worst since a series of major protests followed President Mahmoud Ahmadinejad's controversial re-election in June.
Senior opposition figures have been arrested, and the government has blamed troublemakers for the violence.
On Wednesday, tens of thousands of Iranians gathered in cities across the country in support of the government.
They chanted "death to opponents" as they marched at the rallies, which were reportedly organised by the government in response to the opposition protests.