Defying a government warning, prominent opposition activists today announced the formation in Warsaw of a human rights committee to monitor police abuse.

It was the third such monitoring organization to be set up in Poland in recent days, all spurred by the slaying last month of the pro-Solidarity priest, the Rev. Jerzy Popieluszko, by secret police functionaries.

Although the crime was denounced by Communist authorities and three policemen were arrested and charged with murder, Solidarity supporters hold the government morally responsible, citing failure to discipline police in the past. Independent pressure groups are needed, they say, to prevent recurrences.

Ten days ago, government spokesman Jerzy Urban ruled out the "legal or semilegal existence of such self-styled organizations" as the one announced today. He accused activists of attempting to exploit Popieluszko's death to revive the Committee for Social Self-Defense (known as KOR), which was the leading dissident intellectual group in the late 1970s and assisted in the formation of the independent trade union Solidarity. Both subsequently were banned.

Any repressive move now, when tensions are still high from the shock of Popieluszko's death, could set back government efforts to regain credibility by rigorously investigating the killing. On the other hand, not cracking down risks upsetting the police, communist party members and the Soviets.

Announcement of the Warsaw group came at a press conference held a short distance from the Interior Ministry in the cluttered apartment of Edward Lipinski, a 96-year-old economist.

A founding document said the killing of Popieluszko was only the latest in a series of "mysterious murders, beatings and abductions" that the official Polish press had failed to publicize.

"Our society has a right to the whole truth and we want to serve that truth through our activities," the statement said. "Our society has a right to defend itself against fear and to organize itself against political banditry. Our activities are meant to serve such self-defense."

The declaration was signed by 14 well-known activists, among them former KOR members Lipinski, literary critic Jan Jozef Lipski and writer Anka Kowalska, as well ex-member of Solidarity and mathematician Janusz Onyszkiewicz.

Members of "The Citizens' Committee Against Violence" said it would publicize information on cases of police brutality and harassment and offer legal aid to victims of persecution. Similar committees have been established in the past two weeks in Wroclaw and Krakow.

Poland's opposition movement had appeared to founder after the release last summer of most political prisoners, a move that appeared to leave it unfocused.