The body of the Rev. Jerzy Popieluszko, one of Poland's most celebrated militant priests, was found today by searchers using drag nets in a reservoir at Wloclawek, about 120 miles northwest of Warsaw.

The discovery came 11 days after the charismatic 37-year-old cleric was kidnaped, reportedly by three security police officers who have since been arrested. The kidnaping and subsequent disclosures have shaken the government of Polish leader Gen. Wojciech Jaruzelski.

Confirming the death, which had been widely feared, Polish television this evening said that an autopsy was being conducted and that representatives of the powerful Roman Catholic episcopate were being kept informed of developments.

Throughout the ordeal of Popieluszko's disappearance, church leaders and activists of the banned Solidarity movement have appealed to Poles to show restraint and not be provoked.

In an emotional statement tonight from his home in Gdansk, Solidarity leader Lech Walesa declared sadly, "The worst has happened," but he urged that Popieluszko's death lead to a reconciliation between communist authorities and society and to the start of a new dialogue. "We will all meet at the funeral," he said.

An intensive investigation continued, meanwhile, into who had masterminded the murder. Government spokesman Jerzy Urban told reporters today that there was still no proof of any high-level political or military complicity in the crime. But he hinted at the possibility several times in a prepared statement.

Citing concern for the safety of the three security agents being held on charges of abducting Popieluszko, Urban said the men are under special protection with their guards doubled and their food checked.

The precautions appeared to reflect suspicions that powerful figures in the Polish leadership may try to stop the accused officers from talking. If there were accomplices, Urban observed, they "could be capable of a great deal." Adding to the mystery, Urban also disclosed that the man believed to have led the abduction, Capt. Grzegorz Piotrowski, 33, had recanted a confession that he killed Popieluszko, pushing him into the Vistula River.

Urban said the officer, who is a section chief in the department of the Internal Affairs Ministry that monitors religious activities, is now claiming to have abandoned the priest while he still had a chance of survival.

The other two suspects, Lts. Leszek Pekala and Waldemar Chmielewski, both of whom served under Piotrowski, have said Popieluszko's body was dumped in the reservoir at Wloclawek, 45 miles down the Vistula from the town of Torun, where Popieluszko was ambushed on a wooded road Oct. 19.

Since the announcement last weekend that members of the internal security service were implicated in the kidnaping, Jaruzelski and the minister of internal affairs, Gen. Czeslaw Kiszczak, have been at pains to disavow responsibility for the crime. The suggestion that other senior officials may have ordered the attack against Popieluszko would have ominous political significance if proven true.

Jaruzelski's policy of accommodation with the church and the limited degree of tolerance his regime has shown toward some political opponents have drawn complaints from communist party hard-liners and factions in the security apparatus. But it had been thought that Jaruzelski had consolidated his hold over dissident wings of Poland's power structure.

One theory now gaining ground is that influential establishment elements promoted the elimination of Popieluszko to undercut Jaruzelski or Kiszczak, his close ally. Piotrowski is reported to have told investigators that the kidnaping was motivated by what in his view had been an ineffective response by authorities to Popieluszko's political activities. The other two suspects have said they were just following orders.

But Poles point out privately that it is highly unusual in this country's communist system for functionaries to commit grievous offenses without high-level instructions.

Talking to reporters at a press conference, Urban said the attack had been planned for some time. He said the perpetrators had deliberately covered their own tracks while leaving evidence calculated to lead to the Internal Affairs Ministry. This involved the falsification of official documents, the preparation of alibis and the disposal of incriminating objects.

Among the ruses was an anonymous ransom note that Urban said the security officers have admitted sending to church authorities after the kidnaping.

The suspects have given conflicting and often vague testimony since their arrest last week.

Urban speculated that this may be because they hope to be rescued or protected by someone in authority.

He said "there is no certainty" that others conspired directly or indirectly, but the search for co-conspirators "is of special concern to the investigation."

He added: "The question of who was behind them, who gave the order, why and for what should be answered."

In Washington, the State department called Popieluszko "an eloquent defender of human rights. His death is a blow to the Polish people and proponents of human rights . . . . The perpetrators of this heinous crime must be brought to justice."

Popieluszko got involved in the Solidarity trade union movement at its start in August 1980 as chaplain for striking workers at Warsaw's sprawling steelworks. He maintained an identity with the movement after the declaration of martial law in 1981, conducting monthly patriotic "masses for the fatherland" that drew huge crowds to St. Stanislaw Kostka Church in the northern Warsaw suburb of Zoliborz.

He was frequently the target of official criticism for his antigovernment references and promotion of the Solidarity movement. But Urban today declined to answer a question about how the government had viewed the outspoken priest, appearing careful to avoid critical remarks at this sensitive time.

"I don't think this moment is appropriate for an assessment of the past activities of Rev. Popieluszko," the spokesman said.

Since the kidnaping, Catholic faithful have maintained a vigil at St. Stanislaw's. Word of the popular cleric's death was broken to parishioners there at the end of a mass. A sob arose from the congregation when the officiating priest, Feliks Falegewski, announced: "Father Jerzy is among the blessed today."

He described Popieluszko as "a martyr of love and witness of truth." Referring to him as "a new saint," Falegewski said he envied Popieluszko for having been "blessed by God to die for the life of Poland."

Through the evening, hundreds drifted to the church to share their sadness and pray.

The perimeter of St. Stanislaw's was aglow with mourning candles atop fence posts.