Poland's primate, Cardinal Jozef Glemp, declared today that the killing of the Rev. Jerzy Popieluszko "had plunged society into deep mourning." Leading opposition activists called for the formation of a network of human rights committees to monitor abuses by the communist authorities.

A spokesman for the political opposition, Janusz Onyszkiewicz, warned that the tense and bitter national mood could erupt into anger if authorities attempted a cover-up of those ultimately behind the death of Popieluszko.

Communist officials have hinted that the killing of the fiercely pro-Solidarity cleric may have been part of a broader conspiracy aimed at weakening the position of Poland's leader, Gen. Wojciech Jaruzelski, and his internal affairs minister, Gen. Czeslaw Kiszczak. The party leadership has said it is looking for behind-the-scene instigators in its own ranks.

A spokesman for the Internal Affairs Ministry said the three secret police officers already accused of kidnaping Popieluszko would likely be charged as well with his murder. Poland's main communist daily, Trybuna Ludu, reported today that the captain and two lieutenants had been expelled from the party.

The decision was taken yesterday by the party organization in the ministry. A resolution adopted by the group said the kidnapers "did their best to smear" the entire ministry "with responsibility for this unprecedented crime." It said "the investigation will show whether anyone stood behind this act of terror" and called on all employes of the ministry, which controls uniformed and plainclothed police, to help find "the possible masterminds."

There were no reports of unrest today following last night's television announcement that Popieluszko's body had been found in a reservoir on the Vistula River. The 37-year-old priest disappeared Oct. 19 near Torun.

Pathologists began an autopsy at a forensic medical institute to determine the cause of Popieluszko's death. The Roman Catholic episcopate issued a statement saying a church-approved lawyer and a court doctor had been allowed to witness the examination.

Solidarity leader Lech Walesa appealed yesterday for calm, and other senior activists have managed to quell moves toward demonstrations at major factories, arguing that the authorities must not be given cause to divert public attention from their apparent backstage power struggle.

Funeral services for the defiant priest, now a martyr to millions of Poles who had found inspiration in his anticommunist sermons, are scheduled for Saturday, according to church sources. Arrangements are being handled by the Warsaw curia, an indication that the church plans to hold a large funeral in the capital.

The sources said the government had wanted Popieluszko buried in his native village of Okopy in the Bialystok region in northeast Poland, hoping presumably to avoid a massive turnout. But Glemp resisted. At the same time, the cardinal appears to have rejected an appeal from Popieluszko's friends and parishioners that he be buried inside St. Stanislaw Kostka Church in Warsaw's northern district of Zoliborz, where he served.

Instead, the plan is to hold a mass at the twin-spired, white-stoned St. Stanislaw's, then lay Popieluszko to rest in Powazki, Warsaw's most famous cemetery.

A statement by the primate called for a period of prayer and national mourning. "The death of Father Jerzy Popieluszko, inflicted with cruelty and premeditation, should bear fruit for the church's religious life in prayer, which brings light to complicated paths," Glemp said. "The murder of Father Popieluszko becomes for the church a call for further courageous spiritual work in the nation in accordance with the clerical slogan: 'Don't let the evil overcome you, but overcome evil with good.' "

Expressing indignation over the murder, a group of educators, workers and intellectuals announced in the southwest city of Wroclaw the formation of a committee to monitor human rights.

The organization marks the first attempt by the political opposition to undertake legal activity since martial law in December 1981 crushed and eventually outlawed the Solidarity independent trade union movement.

Organizers told reporters that similar committees would be set up in Warsaw, Gdansk, Katowice and Krakow. Their hope is to see a national network of human rights groups develop.

During a three-hour meeting, participants stressed the need to act quickly, building on public anger in the wake of the abduction and killing of Popieluszko. Among those taking part were Andrzej Gwiazda, former deputy chairman of Solidarity, Jan Winnik, former deputy leader of Wroclaw's Solidarity branch, teachers and doctors, a farmer, a metal worker and a miner.

Some were reported to have raised the possibility that the local committees would merge into a national organization similar to the Workers' Defense Committee, known by its Polish acronym KOR. Formed in 1976, it laid intellectual groundwork for the rise of Solidarity in 1980.

"The police forces have slipped out of any social control and currently even out of control of the political authorities," said a declaration issued by the 24-member Wroclaw group. "The terrorist crime against the person of the Rev. Jerzy Popieluszko has not been the first but it was the most striking example of such actions."

Creation of the Wroclaw committee was welcomed in Warsaw today by Onyszkiewicz, formerly Solidarity's national spokesman, who, in the current crisis, has helped set up a kind of opposition information bureau at St. Stanislaw's.

He told reporters that as above-ground operations, the new monitoring committees will be better able to verify allegations of human rights violations than are underground structures, which have frequently published such claims. "The authorities should know they are under scrutiny by some groups," he said at a press briefing in the yard of the church.

Crowds gathered during the day to pray, light candles and lay flowers at St. Stanislaw's, which has become a shrine to Popieluszko since his abduction. Poems eulogizing the late priest, some signed, others not, have been tacked to announcement boards in the church yard.

Banners clothe the outside and are strung across fences proclaiming "a strike at the heart of the nation" or "defeat evil with goodness" or other messages. A long line formed to buy wallet-sized photographs of Popieluszko at the pulpit.

Asked about the national mood, Onyszkiewicz said the announcement of the death was a turning point. "Something that had been at the back of people's minds was presented as the final truth," he said. "We were shocked. I would say the current mood is tense and bitter."

He said he feared a cover-up of those ultimately responsible for plotting the crime, but he noted that the fact that the authorities had produced Popieluszko's body was "a good sign" of their declared intention to get to the bottom of the incident.