A question of credibility is being put to a federal jury here.

Was an unresisting 23-year-old black man named William Cradle savagely beaten by a gang of policemen until he collapsed in a pool of his own blood next to splinters of shattered nightsticks?

If the jury believes Cradle and a number of whites who testified that they witnessed Cradle being beaten although he was not resisting, three policement will be convicted in the first trial to result from a long series of charges that this city's police routinely brutalize people.

If the jury acquits, Mayor Frank Rizzo and police Commissioner Joseph O'Neill will be reinforced in their position that no cases of improper police use of force have been found.

Rizzo has already explained that "it's very easy to break some of these nightsticks." The mayor, a former police commissioner who has always identified himself with the police, also told reporters: "The bottom line is: nobody will get to them while I'm mayor of Philadelphia."

This is not to say there isn't one area of police behavior Rizzo or O'Neill care about.

None of the 12 men under federal indictment in brutality cases has been suspended, but a policeman was fired for living with a woman to whom he was not married. Rizzo defends keeping indicted men on active duty because they are entitled to a fair trial.

Convicted policemen usually have also been kept on active duty, but four men were ordered dismissed from the force after attending a bachelor party in a strip joint although they face no charges.

The Cradle case, like those centering on Edgardo Ortiz and Robert Wilkinson, which are scheduled to follow, is being tried in federal court on charges that, in savagely beating prisoners, police violated the men's civil rights.

U.S. Attorney David Marston jury last summer after it became clear that no city official was concerned. "There is a blindness to police brutality," Marston said in an interview.

Police brutality has been a fact of life in Philadelphia's black communities for many years, according to black leaders.

In 1972 a Pennsylvania advisory committee to the U.S. Commission on Civil Rights called attention to the problem. "You could change the dates and that report would sound like it was written this year," Anthony Jackson said.

"But people thought it was happening to black folks and they probably deserved it. Now they say, hey, it isn't the criminal, crazy niggers," said Jackson, who heads the federally funded project monitoring police abuse at the Public Interest Law Center of Philadelphia (PILCOP).

"I'm talking about white people beating on white people," said Bob Crucie, a former ward leader in a white area.

Crucie saw a group of men jump out of their car and start beating two other men at 10:30 one Saturday morning recently. When Crucie and others shouted for the police, one of the attackers turned and said; "It's all right, we are the police."

The spread of reported police violence to white, affluent areas has legally been a major factor in changing attitudes here.(Of the three cases, now headed for federal trial, Cradle, although black, was beaten on a street in elegant Society Hill and Wilkinson, who is slightly retarded and served 439 days in prison after a confession to a crime he did not commit was coerced from him, is white.)

Another important factor has been the Philadelphia Inquirer. Reporters William K. Marimow and Jonathan Neumann had each heard testimony about police beating confessions out of suspects while covering several trials. It struck them that beatings might be standard operating procedure, they said.

They ran a computer check of all 433 homicide cases from 1974 through April in which judges were asked to rule on the legality of police methods of investigation. They found that judges had declared that 83 had been illegal.

Marimow and Neumann have been writing about police brutality charges ever since. New cases come to them by phone and mail.

There are now also more than 20 church groups and organizations like Common Cause and the League of Women Voters which are supporting an ordinance that the Coalition Against Police Abuse hopes to get introduced in the City Council next month.

The ordinance would establish an internal police discipline system and what backers hope would be an effective way for citizen complaints to be handled. The present systems produce on results, according to all observers except police officials and city hall officials.

Jackson's PILCOP police project screened the 272 reports of police beatings that came to its attention in 1976 and sent what they felt were the 69 strongest cases to Commissioner O'Neill to investigate. Many of the alleged beatings occurred in the hand-cuff shaped police headquarters known as "The Roundhouse." One policeman was suspended for one day. The other 68 cases were dismissed.

A citizen who acts alone doesn't get much better results. One man who tried to complain about police behavior to the district attorney last Thursday was told to come back Nov. 15. No one was available until then, said James Cassell, news editor of the Philadelphia Tribune to whom the man then took his story. The black-run twice-weekly Tribune has been writing about police brutality for years.

The changed atmosphere in recent months is emboldening more and more people to tell the Tribune of bad treatment by police, Cassell said.

It is also bringing more civil suits and larger judgements against the police. A year ago, said Jayma Abdoo of the Coalition Against Police Abuse, $10,000 was a large award in a suit brought by a beating victim. A woman who was badly beaten recently collected $250,000. She said, "Jurors are reading the newspapers."

The city's taxpayers pay the judgements.

Whether the trend toward greater action to combat police brutality continues or dies down will bear on Rizzo's political future.

The mayor, who is coming to the end of his second term, is widely believed to be preparing to attempt to change the city charter to remove the ban on a third term for Philadelphia mayors. The next mayoral election is November, 1979.

However, Rizzo's candidates for district attorney and city controller were defeated in the Democratic primary in what some observers take as an indication that the mayor's once enormous popularity is dwindling.

All three surviving candidates for district attorney, aware of a new wind direction, are speaking of a need to reduce police brutality although the winner will depend on police to make his cases and the mayor remains unconvinced that brutality is a problem.

Religious leaders are also using their influence to combat police brutality. The Rev. David Gracie has been one of the leaders in organizing attempts to use Philadelphia's pulpits and church letters to speak against abuses by the police.

Rizzo has replied: "I don't tell ministers how to preach the Bible. I don't expect them to tell me how to run a police department."

In the end, no matter who next month's jury believes in the Cradle case, it all comes down to Rizzo.

Almost anyone asked about police brutality in Philadelphia quotes Rizzo's remark when he told an Italian politician earlier this year that the way to treat criminals was "spacco il capo" which means roughly "break their heads."