A coalition of civil rights activists filed suit in U.S. District Court here yesterday in an attempt to stop the FBI from destroying its field office files.

The plaintiffs accused the FBI of carrying on an "indecently stepped-up program" of records destruction intended to undermine the Freedom of Information Act. They charged that field office files contain evidence of FBI break-ins and other illegal domestic activity - evidence they said is not included in the summaries field offices send to headquarters - and they demanded that the files be preserved as a historical record and as a guard against future illegal activity.

FBI spokesman Dave Cassens denied the group's allegations. "We don't have any kind of stepped-up program" for files destruction, he said. "We're not doing anything to undermine the Freedom of Information Act."

In the past four years, information obtained through FBI requests has supported significant charges of illegal FBI activity, including domestic spying such as COINTELPRO.

The suit comes on the heels of FBI Director William Webster's proposals for sweeping cutbacks in the Freedom of Information Act. Webster says allowing individuals to request files from federal agencies impedes FBI investigations by drying up sources who fear their identities might later be disclosed. He wants Congress to put a seven-year moratorium on release of investigatory files, and wants an outright ban on the release of records "collected or used for foreign intelligence, foreign counter-intelligence, organized crime or terrorism purposes."

In his proposals to Congress, Webster denied that he would "use the moratorium provision in concert with a file destruction program to frustrate the Freedom of Information Act."

However, Marshall Perlin, one of the attorneys for the plaintiffs, said that "about 80 percent of the files destruction so far has taken place since the Freedom of Information Act was amended" to strengthen it in 1974.

In 1946, the National Archives and Records Service set the permissible schedule for destruction of FBI files, in time periods ranging from six months for field office reports to five years for unsolved crime files at headquarters. The NARS director of records disposition, Tom Wadlow, said yesterday that it surveyed those limits last year and found them acceptable, but Perlin accused NARS of not keeping adequate watch over the FBI. "You don't ask the fox to watch the chicken coop, and you don't ask the FBI to decide what they're going to destroy," he said.

"We can't destroy any files without specific approval of the archivist," Cassens countered. But archivist Wadlow, when asked if actual destruction of files had been stepped up, said, "You have to ask the FBI that - the agencies implement the schedule."

The 48 plaintiffs in yesterday's suit include Daniel Ellsberg, Angela Davis, the American Friends Service Committee, the American Indian Movement, and Nation magazine. Perlin, who is counsel for Robert and Michael Meeropool, the sons of Julius and Ethel Rosenberg, who were indicted for treason and executed in 1953, said he originated the suit because of difficulty he had getting information on the Rosenberg case from the FBI.

"We're concerned about future Rosenbergs," Michael Meeropool said. "We've already discovered evidence of surveillance, break-ins, attempts to denigrate and smear other organizations and individuals."