The 54 are continuing to collect more signatures to their commitment among reformist colleagues in the 290-seat parliament who are facing wholesale rejection by a conservative candidate-vetting body.
They also vowed in a statement not to sit in the outgoing parliament between the 20 February elections and the opening of the new assembly in June, potentially paralysing the legislature for months.
"We, representatives of the people, swear that if the rights of the candidates in the legislative elections are not guaranteed we will not take part in elections where the free choice of voters is trampled," the statement said.
"We bind ourselves not to sit in parliament ... if the elections amount to nominations," it added.
Earlier on Thursday, Iranian hardliners beat up speakers at a protest meeting against the wholesale rejection of pro-reform election candidates, hours before the expiry of a reformists' ultimatum warning of a poll boycott.
Some 200 members of the Islamic Hizb Allah movement burst into the meeting in the ultra-conservative town of Hamedan in western Iran late on Wednesday, the state news agency IRNA reported on Thursday.
"We, representatives of the people, swear that if the rights of the candidates in the legislative elections are not guaranteed we will not take part in elections where the free choice of voters is trampled. We bind ourselves not to sit in parliament ... if the elections amount to nominations."
Speakers were condemning the decision of the conservative Guardians Council to ban hundreds of reformists from standing in 20 February parliamentary polls for allegedly failing to satisfy the Islamic qualifications.
The assailants injured a number of speakers, including student leader Said Razavi Fagih, reformist MP Husein Lughmanian and Hussein Mujahid, a head of the main pro-reform party, Islamic Iran Participation Front (IIPF), IRNA said.
The reformist daily Yas-e No reported that Mujahid was hospitalised with his nose and an arm broken.
The 12-member Guardians Council which screens all legislation and candidates plunged Iran into one of its most serious crises when it disqualified 3605 of the 8157 people seeking to stand for the parliament, or Majlis.
Most were reformists. Among those figuring on the blacklist were prominent figures in the reform movement and some 83 incumbent MPs.
Supreme leader Ayat Allah Ali Khamenei last week ordered the council, all of whose members he directly or indirectly appoints, to be less stringent in its vetting procedure.
The council has since reinstated some 300 candidates, but none of them is a sitting MP.
The list of approved candidates
will be released on 30 January
On Sunday, 18 reformist parties in a coalition said in an open letter to President Muhammad Khatami that they would decide on Thursday whether to boycott the election.
The coalition, led by the IIPF of Khatami's brother Muhammad
Riza Khatami, said it would make its decision based on the extent of the Guardians Council's review process.
Several dozen reformists MPs were on Thursday continuing a sit-in protest which was launched on 11 January when the Guardians Council issued its blacklist, triggering charges of an attempted "coup" in the Islamic republic.
"So far, the files of 260 (rejected) candidates have been
approved," said Sayid Muhammad Jahromi, an election official in the council, quoted by the state television.
He said 10 commissions have been set up to re-examine the
vetting process and that their decisions would be "announced
gradually over the coming days".
Despite the ultimatum laid down by deputies, the Guardians
Council has until 30 January to inform the interior ministry, which is in charge of organising the polls, of the final approved list of candidates, he said.