Reformist lawmakers have broken a taboo and publicly challenged Iran's supreme leader Ayatollah Ali Khamenei over the country's political crisis.
The Supreme Leader's word is normally final and incontestable
The MPs demanded the ayatollah explain his role in the ban on thousands of candidates in this week's elections.
The bans were made by the right-wing Council of Guardians, whose 12 members are appointed by Ayatollah Khamenei.
Analysts say the move, by MPs who have resigned from parliament, is an extraordinary break with protocol.
They questioned the leader defiantly on several key points that he has made during the debate over the disqualifications.
The BBC's Jim Muir, in Tehran, says the ayatollah essentially gave up trying to persuade the Council of Guardians to back down and reinstate more of the banned candidates.
Did the council dare to resist his orders, the letter asked, or could it be that he had said one thing in public and told the council something else behind the scenes?
Was there any explanation other than that the leader had approved the council's illegal actions when he insisted that such a sham election should go ahead on schedule?
Our correspondent says the supreme leader's word is normally final and incontestable on any subject, so this was an extraordinary and daring move by a group of disillusioned politicians who clearly feel they have little more to lose.
It is not clear how many of the 120 or so resigning deputies did sign the letter.
New poll blow
News of the move came as a leading Iranian reformist MP challenged President Mohammad Khatami's call for people to vote.
Reza Yusefian - one of 80 sitting MPs banned from the polls - told the BBC that it was unfair to ask people to vote with no real competition for the hardliners.
Mr Khatami made his plea in the hope of stopping conservatives winning full control of parliament, observers say.
The poll suffered a new blow last Saturday with the withdrawal of a further 550 candidates who had been deemed suitable to stand.
The biggest pro-reform faction, the Islamic Iran Participation Front, had most of its top leaders disqualified and is not taking part.
About 46 million Iranians are eligible to vote in the elections, but observers say there is little sign of public interest in them.
Mr Yusefian told the BBC World Service's World Today programme: "We are not convinced why the president has urged the people to participate in the election and we are not satisfied with his call."
"I think that the MPs' sit-in and resignations have convinced a lot of people that the reformists are serious regarding their demands for heading towards reform and they are not going to make some compromise and get some privileges for themselves," he said.
Mr Yusefian's comments appeared at odds with Mr Khatami's call to voters to pick candidates with views closest to their own.
Mr Khatami disappointed many reformists who hoped he would refuse to endorse the 20 February elections unless they were free and fair.
A government survey predicts a turnout of about 30% of voters nationwide but some experts say it could fall as low as 10% in cities.