MANILA, Philippines (AP) _ Anti-riot police once loyal to Ferdinand E. Marcos fired into a stone- throwing crowd of the ex-president's supporters Monday. One person was killed, hospital officials reported.

They said some 60 people, including 18 policemen, were injured in the fighting at the City Hall in the Manila suburb of San Juan.

The battle occurred several hours before pro-Marcos legislators met in a Manila office building and declared they had reopened the abolished National Assembly.

In the first major confrontation between police and demonstrators opposed to President Corazon Aquino, 60 police charged into nearly 1,000 people blockading the San Juan City Hall.

The demonstrators were protesting the government's ouster of Joseph Estrada, a movie star and Marcos loyalist, as the mayor of San Juan.

A police official claimed there was shooting from the crowd, but the official police report did not mention any guns in the hands of demonstrators. Reporters saw some plainclothes officers in the police lines armed with rifles.

Mrs. Aquino, at a meeting with military commanders Friday, asked them to show maximum tolerance for the demonstrators, according to presidential spokesman Rene Saguisag.

Meanwhile, the chief prosecutor in the case involving the 1983 assassination of Mrs. Aquino's husband, ex-Sen. Benigno Aquino, formally accused Marcos of interfering with the trial. Prosecutor Manuel Herrera made the allegation in an affidavit filed with the Supreme Court, which is considering overturning the verdict acquitting 25 military officers and one civilian.

Aquino, considered Marcos' main rival, was slain at the Manila airport as he returned from self-exile in the United States.

About 2,000 pro-Marcos people gathered in front of the Asian Institute of Tourism where ex-legislators from Marcos' New Society Movement met and declared they had reopened the National Assembly, which Mrs. Aquino abolished after she came to power.

They cheered as Arturo Tolentino, Marcos' vice presidential running mate in the Feb. 7 election, entered the building.

Some of the Marcos supporters used stones and sticks to chase away a smaller pro-Aquino group that approached the area.

''We're still for Marcos,'' the crowd chanted in the Tagalog language, waving banners that said: ''We want the legal president back'' and ''God chose Marcos and Tolentino.''

Pro-Marcos demonstrations have increased in recent days, with 15,000 people at a downtown rally Sunday and several hundred providing a defensive blockade for a radio station that said it received threats after carrying a live interview with Marcos.

Some of the Marcos loyalists claim the allegations that the ex-president stole billions of dollars in government funds were propaganda from the Aquino government.

But little was said about Marcos in the meeting attended by 93 members of the abolished 190-seat parliament.

Tolentino, 76, did not take the oath of office when Marcos had himself sworn in for another term just hours before he fled his palace for exile in Hawaii.

But Tolentino says he is the legal vice president, and officials of the New Society Movement said Tolentino will have a public oath-taking in a few weeks.

Tolentino, in a speech to the rebel assembly, called for civil disobedience against Mrs. Aquino's government - a tactic she advocated before taking over when Marcos fled Feb. 25 during a civilian-military revolt.

''If the people in the past resorted to civil disobedience, now I say also, 'Exercise civil disobedience,''' said Tolentino, claiming Mrs. Aquino's government is illegal.

No pro-Aquino assemblymen attended the meeting, although many had criticized the new president for her abolition of the assembly to clear the way for a 50-member commission she appointed to write a new constitution.

Among the resolutions passed by the rebel assemblymen was one calling for local mayors and governors to remain in office.

Saguisag, asked about the meeting, said it was not illegal and came under the protection of freedom to assemble. ''As far as we're concerned, they're just having a reunion,'' he said.

In San Juan, the fighting left the streets littered with stones, sticks, damaged vehicles and debris from inside the ransacked City Hall.

Estrada had surrendered his post Friday after holding out against Mrs. Aquino's government for three weeks. He was replaced as mayor by Aquino supporter Reynaldo San Pascual.