Last Sunday, with the Redskins-Lions football game on the radio in the background, about 30 staff members of the Iran-contra investigating committees sat in a room at the Government Printing Office on North Capitol Street proofreading their final report at the last minute.

The 690-page tome released yesterday already is being praised, attacked and dissected around Washington. And while House and Senate committee members faced the spotlight in news conferences once again, bleary-eyed staffers paused for a champagne toast and recollections of their efforts.

Since the public hearings ended last summer, the staffs have been busy boiling down one million pages of documents and the testimony of 250 witnesses to what they hope will be a historic document.

The report gives new meaning to the phrase "written by committee," participants in the process said yesterday. The 427-page majority report has 28 chapters and each underwent several revisions. They were passed between the committee staffs and members several times before a professional editing team, headed by Joseph Foote, began production.

"The report is truly a joint effort," said Lance Morgan, press officer for the Senate committee and one the report's several editors. "It's meant to be read."

John W. Nields Jr., chief counsel of the House committee staff, said he thought the constant rewriting improved the report. He rejected the caustic comments of eight Republican members who defected to write the 155-page minority report.

Some GOP staff aides, who declined to be quoted by name, said they heard comments along the way, especially from Arthur L. Liman, chief counsel of the Senate committee, that Liman was intent on shaping the final report as a political attack on the Reagan administration.

"The report will speak for itself," Nields said. "I think we wrote it factually, and the conclusions were driven by the facts."

Morgan noted that the Senate vote for the majority report was 9 to 2, and included three Republican senators, committee Vice Chairman Warren B. Rudman (N.H.), William S. Cohen (Maine) and Paul S. Trible Jr. (Va.).

Staff members interviewed yesterday generally praised the cooperation of the White House declassification team, headed by Brenda Reger, that worked to ensure the 2,100 pages of double-spaced typed material that became the printed report did not inadvertently disclose secrets. "They didn't delay the process at all," Morgan said. "We buried those people in paper and they were magnificent."

The administration did balk on some committee requests to declassify material. For example, Patrick J. Carome, a House staff counsel, said he hoped to write chapters about the Iran arms sales using the names of countries involved in the transfers. Instead the report refers to them by number, listing "Country 1" to "Country 18."

Some members complained in views supplementing the report that other White House officials did not cooperate as promised in filling requests for documents. And the report noted that the White House balked at furnishing backup computer tapes that might contain additional memos that National Security Council staffers such as Lt. Col. Oliver L. North thought they had deleted from the system.

The two committees were forced to hire outside computer experts to write a program to check for missing documents and still expect the administration to run the tests, Nields said yesterday. He added that he feels there is "a very low probability" that anything significant will be uncovered by the exercise.

The report, which is on sale now at GPO bookstores for $29, will be available soon as a commercially printed paperback book, committee officials said yesterday. In addition, the committees are still working to declassify scores of depositions from witnesses. And Senate staff attorney Isabel McGinty is completing a chronology containing excerpts of testimony that may be as long as the final report, Morgan said.

The 20-page executive summary, which is set in larger type than the rest of the report, went through the most revisions because staffers and committee members realized it would be closely read. One Senate staff aide said Liman, and Mark A. Belnick, his top assistant, supplied much of the sometimes eloquent rhetoric, while Nields and his House colleagues furnished the facts to bolster the contentions.

The writing began last August, with attorneys from each committee drafting chapters and then giving them to their counterparts to critique. For example, Carome wrote drafts of what became Chapters 10 and 11, covering the U.S. arms sales to Iran, while other staff workers wrote sections on such topics as resuppling the Nicaraguan contra rebels, private fund-raising for the contras, and the money trail, according to House committee spokesman Bob Havel.

Once the staff attorneys were finished, Liman and Nields and their assistants began editing and rewriting. The first complete draft was ready for committee members to read by mid-September.

Nields said that there were substantive disagreements between the two committees' staffs on a few issues. But he said he felt about 90 percent of the debate with the Senate was over how to write a conclusion, rather than what the conclusion should be.

Morgan said the Senate committee has spent about $2 million. Havel said the House inquiry had spent about $1.2 million through Sept. 30. The committees officially go out of business Dec. 13.