A special federal grand jury will spearhead a broad-based investigation of alleged systematic beatings of witness and criminal defendants by Philadelphia homicide detectives and uniformed officers.

The special grand jury was sought by U.S. Attorney David W. Marston, who has assigned four prosecutors and 15 FBI agents to conduct the massive probe of the Philadelphia Police Department under federal civil rights laws.

Marston's unusual requests for a single-purpose grand jury was approved by U.S. District Court Chief Judge Joseph S. Lord III, who said, "According to the reports by the U.S. attorney and what I have read in the newspaper, it was something that needed to be investigated."

He was referring to articles published in the Philadelphia Inquirer since December describing police brutality, largely by members of the homicide division.

Marston began to probe in December after the Inquirer reported police beatings of witnesses in a firebombing case. The statement thus coerced from the witnesses led to the conviction of an innocent man in the burning deaths of four members of a north Philadelphia family and a friend The man was later exonerated and released from prison.

The federal probe expanded dramatically in April when the newspaper tabulated 80 homicide cases over a three-year period in which local judges ruled that the police acted illegally during interrogations. A series of investigative articles by Jonathan Neurmann and William K. Marimow contained information documented in court records:

"The suspect or witness is often handcuffed to a metal chair, which is bolted to the floor. Some of these sessions have lasted 24 hours."

"Techniques include placing a telephone book on a suspect's head and beating his feet and ankles; twisting or kicking his testicles and pummeling his back, ribs and kidneys."

"Testimony has shown 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."

"Suspects and witnesses have testified that they were forced to watch beatings through (one way) windows and were told they would receive the same treatment unless they cooperated."

"What we're living in at the Roundhouse (police headquarters) is a return to the Middle Ages," one former homicide detective told the reporters.

City Hall reaction to the stories and the federal inquiry has been supportive of the local police. Mayor Frank L. Rizzo, whose career began in the Philadelphia police department as a patrolman and reached police commissioner said, "I'm sure the police acted properly" and that he "welcomes" any investigation.

"I'm sure we have overzealous policemen, jsut like we have overzealous reporters," said the mayor in a slap at the Inquirer. He has a $6 million libel suit pending against the newspaper, which published a satirical column featuring Rizzo last year.

"We have a very, very good police department and they're not brutal," he said, "It's very easy to break some of these nightsticks," he noted when asked to comment on one incident in which police broke two nightsticks during a beating.

Behind the scenes, city solicitors have been fighting federal subpoenas served on the police department, and state House Speaker K. Leroy Irvis reported that Rizzo has told him of his opposition to subpoena power for a sparate probe by the House Judiciaty Committee's subcommittee on crimes and corruption.

Police Commissioner Joseph O'Neill, the taciturn successor to Rizzo, refused to make any comment on the allegations or investigations.

A statement by Thomas A. McCarey, president of the Philadelphia lodge of the Fraternal Order of Police, said, "there is an innate bias (against the police) in the series as presented so far," bu failed to point out any factual errors in the reporting.

Ironically, the fate of Marston as U.S. attorney in Philadelphia has hung in the balance since the installment of the Carter administration. Marston, a former aide to U.S. Sen. Richard Schweiker (R-Pa.), has refused to offer his resignation to the Democratic administration.

Philadelphia's Democratic congressional delegation and the Rizzo administration have lobbied for the Carter administration to replace the agressive prosecutor with a Democrat. Marston's most recent successful prosecution was the conviction of Herbert Fineman, a long-time powerful Phildelphia Democrat, in a medical school admissions payoff scheme. Other powerful Democrats are targets of Marston's political corruption investigators.

As for police brutality, Marston said, "People have no place to turn in these kind of cases.

"The local district attorney can't deal with them. It's up to the U.S. attorney to investigate police brutality. I'm convinced, though, that of the 8,000 members of the Philadelphia police department, a vast majority of them are obeying and enforcing the law."