The second of four secret policemen scheduled to testify in a trial for the murder of pro-Solidarity priest Jerzy Popieluszko portrayed himself today as a reluctant participant in the crime, saying a severe beating of the priest had been "like a nightmare."

Asked by a judge why he had not pulled out of the assassination, former lieutenant Waldemar Chmielewski said, "I don't know. I can't explain. It is hard to explain."

Showing the strain of a second day on the stand, Chmielewski, who spoke with a bad stammer, which he has said began after the killing, appeared pale and exhausted by midday, prompting the court to summon two physicians. After examining the defendant, the doctors recommended that recesses be called every hour and Chmielewski be allowed to sit while testifying. The five-judge panel consented.

According to several observers attending the restricted trial, Chmielewski appeared more agitated today than previously, rubbing his eyes and tugging at his collar. Yet he sounded more articulate and thoughtful than the other former lieutenant, Leszek Pekala, also charged in the case, whose testimony occupied the first three days of the unusual hearing.

Chmielewski's symptoms struck some observers as suggesting a greater depth of pain or fear than his colleagues have shown. He avoided the automatic response Pekala had given to explain his involvement in the killing -- that they were just following orders -- and appeared troubled by what it was that had driven him to join in a slaying he and Pekala have said was not intended.

Also indicted for murder is their immediate superior, former captain Grzegorz Piotrowski, whom both lieutenants have described in often vivid testimony as the driving force behind the killing. Facing a charge of aiding and abetting in the crime is the deputy director of the Interior Ministry department in which the three officers served, Col. Adam Pietruszka.

In an unrelated case, a senior member of the outlawed Solidarity trade union said today that he had received a summons for a civil action by the state aimed at recovering 80 million zlotys (about $580,000) that Polish authorities allege were embezzled from Solidarity's coffers.

Jozef Pinior, who was the chief financial officer of Solidarity's Lower Silesian chapter, has admitted withdrawing the funds 10 days before martial law was declared on Dec. 13, 1981, but said in a statement that he views the money as belonging to the now-illegal union and claimed the government has no right to ask for an accounting of it. The money is believed to have been used to finance underground political operations.

Also named in the suit, which is scheduled to begin in Wroclaw Monday, is another Solidarity leader from the southwest region, Piotr Bednarz. Both Pinior and Bednarz were pardoned of criminal responsibility for taking the funds under a broad amnesty last summer granted to political and other prisoners.

Chmielewski, under questioning by Judge Jurand Maciejewski, recounted in detail two assaults on Popieluszko by Piotrowski, Pekala and himself. In one effort Oct. 13, the three secret policemen tried to stone the windshield of Popieluszko's car.

A second attempt on Oct. 19 led to Popieluszko's abduction and death. His body was found 11 days later in a reservoir northwest of Warsaw at Wloclawek.

In the six days following the first attack, Chmielewski joined the other two in preparing for another attack on the outspoken cleric. He obtained a militiaman's uniform, which he wore later to wave Popieluszko's car off the road, and with Pekala he stitched some sacks into a large bag in which they intended to place Popieluszko.

For the first time the name of Piotrowski's deputy, Janusz Drozdz, was mentioned in courtroom testimony, although his connection with the case was left unclear. Chmielewski quoted Piotrowski as saying at a meeting after the Oct. 13 stoning, that actions against Popieluszko were being "carried out on the broadest scale, including actions by other people." Among those named, said Chmielewski, were Drozdz and the Warsaw militia headquarters.

The indictment also cites Drozdz as participating in meetings in september and October at the Interior Ministry at which various schemes against Popieluszko were discussed, including one in which the priest "might fall out of a train." This was subsequently dropped as unfeasible, the document says.