According to the student news agency ISNA, the Guardians Council rejected 877 of the 1700 prospective candidates who had registered in the capital to stand for a seat in the Majlis, or parliament on 20 February.
The agency also listed a number of key pro-reform figures it said had also failed to get past the highly contested vetting procedure, including Muhammad Reza Khatami.
He is a brother of the reformist president, an incumbent MP and the head of the Islamic Iran Participation Front (IIPF), Iran's largest reform party.
Two other top IIPF members, Behzad Nabavi and Fatimah Haqiqatjou, were also barred from standing by the electoral vetting arm of the Guardians Council - an unelected senate-like body that has also blocked most Majlis legislation since reformers took control of parliament in 2000.
In addition, Muhammad Reza Khatami and Behzad Nabavi are both the current deputy speakers in the Majlis. However, the report said the candidacy of the present Majlis president, Mahdi Karubi, was approved.
"We will defend the rights of all those whose candidacies have been rejected"
Also reportedly rejected was Muhsin Mirdamadi, an outspoken reformer and current head of the Majlis foreign policy and national security commission.
Outspoken leftist Muhsen Armin and top women's rights activist Elaheh Koulaiee - both also incumbent deputies - were also barred from standing again.
ISNA said members of the liberal Iran Freedom Movement (IFM) and so-called religious-nationalists were also barred from being candidates.
"I regret and I am saddened by this massive rejection of the forces inside the parliament and personalities from outside parliament," Karubi said during Sunday's Majlis session carried live on state radio.
"I am in contact with President Khatami, and for several days we have worked together and held discussions. We have spoken to the Guide (supreme leader Ayat Allah Ali Khamenei) and the Guardians Council, and we will continue," he added, calling on those rejected to "lodge a formal complaint".
"We will defend the rights of all those whose candidacies have been rejected," he asserted, adding that the Guardians Council's move had yet to receive the formal approval of the supreme leader.
Reformists plan sit-in
Several reformist MPs were poised to stage an all-night sit-in at the national parliament to protest against the move to bar large numbers of reformers from standing for election next month, a prominent MP said on Sunday.
"Reformist MPs will stage a sit-in in parliament this evening. They will stay there until there is a change in attitude over candidacies," Elaheh Koulaiee, a female reformist MP and women's rights activist, said.
Koulaiee's candidacy was also rejected by the Guardians Council.