On a cool damp spring night two years ago, more than 20 Philadelphians watched in stunned disbelief as 10 policemen beat a black man, breaking nightsticks on his head and shoulders, after he had run a stop sign.

A few days later, in early May 1977, Philadelphia Mayor Frank L. Rizzo said this about the incident: "It's very easy to break some of these nightsticks nowadays."

Seven months later, six Philadelphia homicide detectives were convicted in an unrelated case in federal court for beating and intimidating eight people during interrogations leading to the wrongful murder conviction of an innocent, mentally retarded white man.

After the detectives' convictions, Rizzo announced that the officers would remain on active duty "as long as I'm the mayor." He declared: "These were good men who did nothing wrong."

So it came to no surprise to Philadelphians yesterday that the Justice Department had filed an unprecedented lawsuit charging the city and the mayor with allowing its police to beat citizens so freely that it "shocks the conscience."

Nor was Mayor Rizzo's reaction to the suit a surprise. Asked about the extent of police brutality in the city, Rizzo said: "I've never seen any."

For the past 12 years, Rizzo, first as police commissioner and since 1972 as mayor, has guided the Philadelphia police on an unusual crusade. He has made them, in his own words, "the toughest cops in the world."

Rizzo started out as a "law and order" man in the 1960s, but recently has said he doesn't have much faith in the law, pointing to Supreme Court rulings that he said have made police work more difficult and the indictment and arrest of more than 30 Philadelphia policemen by state and federal authorities.

Since Rizzo has been mayor, police department statistics show that more than 150 civilians have been killed by policemen. In more than half those cases, the civilians were unarmed when they were beaten or shot.

In none of those cases were the policemen suspended or disciplined publicly. Only one case resulted in a conviction, and that after a guilty plea by the policeman. The officer resigned and was sentenced to probation.

A typical shooting case is being heard in a Philadelphia court this week.

Last Sept. 23, Cornell Warren, a young black man, was arrested for reckless driving. He and a passenger in his car were handcuffed and taken to police headquarters for questioning.

When they arrived at the station, a quarrel began between Warren and policeman Thomas Bowe. There is a dispute as to who started the argument.

But witnesses and Bowe agreed in testimony that Warren, whose wrists were handcuffed behind his back, ran from the police van, fearing that he would be beaten by police.

Bowe ran after Warren and caught up with him a block away, just outside the Afro-American Museum. There, Bowe shot Warren in the head.

Evidence still is being presented in Bowe's murder trial. The defendant remains on active duty as he sits in the courtroom each day.

While the jury is still out on a number of shooting and beating cases in Philadelphia, another area of violent police conduct under the Rizzo administration already has been well-documented.

According to court findings in pretrial hearings, Philadelphia's system of interrogations routinely includes intentional violence and coercion by detectives.

In 1977, the Philadelphia Inquirer published the results of an investigation that found a pattern of beatings, threats of violence and knowing disregard for the Constitution in interrogation of homicide suspects and witnesses.

The study, which included lengthy interviews with homicide detectives, showed that many policemen, in beating suspects and later denying it under oath, had come to accept breaking the law as part of their job.

"What we're living in," one former detective said, "is a return to the Middle Ages.... Police are breaking the law every day and they know it."

From 1974 to 1977, judges ruled that Philadelphia homicide detectives acted illegally in 80 separate interrogations. According to court findings, the illegal interrogations followed this pattern:

They were conducted by teams of detectives in tiny rooms at police headquarters.The suspect or witness often was handcuffed to a metal chair bolted to the floor. Some of these sessions lasted 24 hours.

Some of the techniques used in the beatings left little mark. They included placing a telephone book on a suspect's head and hammering it with a heavy object, beating his feet and ankles, twisting or kicking his testicles and pummeling his back, ribs and kidneys.

Other techniques did leave marks. Testimony about interrogations that judges ruled illegal shows that suspects have been beaten with lead pipes, blackjacks, brass knuckles, handcuffs, chairs and table legs. One suspect was stabbed in the groin with a sword-like instrument. Another testified he was almost suffocated when a detective wrapped a plastic bag over his head while he was handcuffed.

The detective used one-way mirrors to observe the interrogation rooms. Suspects and witnesses have testified that they were forced to watch beatings through such windows and were told they would receive the same treatment unless they cooperated.

Why were detectives doing this?

The policemen explained in interviews that their main personal motivation is outrage - outrage at the heinous crimes they investigate and outrage at a court system that allows murderers to "walk," or go free.

"It's a fight every day," one detective said. "The homicide detective must fight the lawyers, the judges, the Supreme Court - and he must fight crime."

Philadelphia's policemen face the same frustrations all big-city cops face, but they, unlike most other police, feel freer to say what is on their minds and to vent their frustrations on suspects and witnesses.

The reason, they say, is that they are not only allowed but encouraged to be violent. The word comes from the top.

Rizzo, who began his career as a beat cop in October 1943 and worked his way up to police commissioner by 1967, generally does not deny specific charges against police.

Instead, he calls the charges "a farce" or says they are politically motivated.

Rizzo's attitude was exemplified in a trip to Rome, where he invited the Italian senate president to send 10 policemen to Philadelphia to study police methods.

"We'll show them how to eat those 'em 'up. Grind 'em up." guys up," Rizzo boasted. "We'd grind

The mayor said the way to deal with criminals is "spacco il capo," which means "break their heads."

Rizzo is finishing his second fouryear term as mayor, and under city law, he cannot run for reelection this year. (He tried to change the law through a referendum last fall but was defeated two to one.)

While he may be on his way out as mayor, Rizzo boasts that he has kept his campaign promise in his eight years in office:

"I'm gonna be so tough as mayor," he vowed eight years ago, "I'm gonna make Attila the Hun look like a fagot."

In April 1977, the U.S. attorney's office in Philadelphia began a full-scale investigation of police violence. By October of that year, then-U.S. Attorney David Marston had announced the indictment of 15 policemen.

(Nine were convicted, three acquitted and charges against three others dismissed pending a further probe.)

A special grand jury was investigating about 75 cases. Just as the probe was gathering momentum, Marston was fired by Carter.

The new U.S. attorney, Peter Vaira, has not announced any indictments of police.

At the same time, however, the city elected a new district attorney in November 1977. D.A. Edward Rendell opened a "police brutality unit," which investigated only charges of illegal police violence.

The unit has arrested about 20 policemen, including three officers filmed by a television crew while beating and kicking Delbert Africa in a confrontation with police last summer. That film was shown on the CBS Evening News, with close-ups of police kicking Africa's head like a football. CAPTION: Picture, "Complete hogwash" was Rizzo's response to Justice Department suit. UPI