PHILADELPHIA (AP) _ Frank L. Rizzo, the brash cop-turned-mayor, collapsed and died Tuesday while working to recapture his old City Hall job. He was 70.

Rizzo, a Democratic mayor for two terms who was the surprise winner of last spring's Republican mayoral primary, died at Thomas Jefferson University Hospital, doctors said. The cause of death wasn't known, said hospital spokeswoman Cindy Hoffman.

Rizzo collapsed at lunchtime in the washroom of his downtown campaign headquarters, where long-time friend Tony Zecca found him. Rizzo was taken by ambulance to the hospital, where doctors were unable to revive him.

''I think part of Philadelphia died with him today,'' said Zecca, who made the formal announcement at the hospital.

Karen Warrington, press secretary to Mayor W. Wilson Goode, said: ''The mayor extended his deepest sympathy and condolences to the Rizzo family.''

Rizzo's death threw the November election into uncertainty.

Current state election law gives the Republican City Committee the power to select a substitute nominee to appear on the Nov. 5 ballot. That law, however, could be affected by a federal court challenge of the appointment of candidates to run for the seat of the late Sen. John Heinz.

Rizzo was running against Democrat Edward Rendell, a former prosecutor.

''All Philadelphians, whatever their political affiliations, will feel his passing as a tremendous loss and we will all miss him,'' Rendell said.

At Rizzo campaign headquarters, workers answered telephones and talked quietly about their boss, who had marched into the office to greet them a few hours earlier.

''The last time I saw him was when he passed,'' field worker Charles Harris said. ''He always spoke to everyone. Everyone.''

Rizzo was a street cop who rose to become police commissioner during a 38- year career with the department.

Dubbed the ''Cisco Kid'' and the ''Bambino,'' he had a commanding physical presence, standing 6-foot-2 1/2 and weighing 250 pounds. During the riots of the 1960s, he left one black-tie affair to go to a riot scene. He arrived with a nightstick tucked into his tuxedo cummerbund.

''He was one of the best friends of the police,'' said John Shaw, president of the city's police union, the Fraternal Order of Police.

''He had a charisma. He didn't pull any punches,'' Shaw said. ''What you saw was what you got.''

Rizzo was elected mayor in 1971 and re-elected four years later.

During a stormy mayoral career, he directed the police department in its first confrontation with the radical group MOVE during the late 1970s. One policeman was killed when police stormed the house and bulldozed it. Several MOVE members were later convicted and imprisoned.

He unsuccessfully tried to change the city charter so he could serve a third term. In 1983, he lost the Democratic primary to Goode. Four years later, he switched parties and won the GOP primary, but again lost to Goode in the general election.

In this May's primary, Rizzo beat the GOP-endorsed candidate, Ron Castille, and another contender by about 1,400 of 130,000 votes cast. The city charter prevents Goode from seeking a third term.

Francis Lazzaro Rizzo was born Oct. 23, 1920, in south Philadelphia, the oldest son of an Italian immigrant.

After dropping out of high school, serving in the Navy and working at a steel plant, Rizzo joined the police force at age 22.

He rose steadily, and gained notice in the 1960s when he led police raids on gambling houses, after-hours clubs and houses of prostitution. He became commissioner in 1967.

He was criticized for a secret police squad that investigated his political enemies; and a lie-detector test he agreed to take over a secret political deal - and flunked. He also was blamed for a deterioration in race relations. However, he would rebut such accusations, saying he was a mayor for all the people.

He and his wife of nearly 50 years, Carmella, had two children.

A funeral was scheduled for Friday at the Cathedral Basilica of SS. Peter and Paul.