Displaying the Philippine government's tougher new policy on street protests, the police here today used tear gas to break up two demonstrations, one of them aimed at the U.S. Embassy.

It was the first time in the current series of protests that the authorities stepped in to quell demonstrations without first being attacked by protesters.

Later tonight, police arrested more than 20 persons in the streets not far from the presidential palace where loosely organized gangs of youths gathered to throw rocks and bottles at soldiers and police. A passenger in a taxi was shot in the back by police when the cab driver inadvertently ran through a police roadblock.

The new tactic of snuffing out protests with tear gas appeared to reflect the toughened posture of President Ferdinand Marcos--against whom most of the protests have been aimed--since the assassination of his chief rival, Benigno Aquino Jr.

Marcos implicitly threatened to invoke martial law yesterday after a night of severe violence near his official residence and warned his political opponents not to force him to "extremes."

At one demonstration today, about 2,000 persons from the downtown university district came within 200 yards of the American Embassy, which looks out over Manila Bay.

Police lobbed tear gas at the crowd near an intersection, and the protesters fled into a nearby entertainment district. Some in the front of the march were clubbed by police.

At a campus gathering spot, the students had prepared an effigy of Uncle Sam with the name "Armacost" scrawled on it. Michael Armacost is the U.S. ambassador here.

There have been anti-American demonstrations at the embassy in the past, but this was the first one since a series of major rallies began a week ago. Many leftist students and some in the political opposition criticize the American policy of supporting Marcos.

In another development, Cardinal Jaime Sin, archbishop of Manila, met with Marcos and again urged him to join in forming a "council of reconciliation" that would work out a plan for healing national division.

A spokesman for the cardinal, who frequently criticizes Marcos, said the proposal was "in a more concrete form" than when Sin had first proposed it in February.

The spokesman quoted the cardinal as saying that Marcos had promised to "take a look at it."

Earlier, police used tear gas to break up an impromptu street gathering in Makati, which is the city's main business and financial district.

The rally there had begun within office buildings overlooking the main thoroughfare, Ayala Avenue. Streamers and confetti cut out of telephone books fell from windows along several blocks while office workers shouted "Ninoy," the nickname of the slain Aquino.

Eventually, a large crowd gathered to mill around and join the protest in Ayala Avenue and traffic came to a stop. Police wearing face masks appeared in pickup trucks and hurled tear gas canisters to disperse the crowd.

In neither of the two rallies had the protesters tried to assault police, unlike the major Wednesday night confrontation that started when a group of demonstrators hurled rocks, bottles and home-made explosives at police and soldiers.

But tonight, rocks and bottles were thrown by the disorganized gangs that gathered on street corners in the vicinity of Malacanang Palace, the official residence of Marcos.

Lines of police with shields guarded several intersections and made repeated forays to force the crowds back. They were backed up by troops, some of whom prominently displayed automatic weapons. In previous encounters, police had initially appeared on the streets unarmed.

Anti-Marcos rallies in the financial district of Makati began one week ago and have added a new dimension to the protest movement since Aquino was shot to death at Manila's airport.

The rallies there have included businessmen, professional people and office workers who had never taken part in demonstrations before. An attempt earlier this week by pro-Marcos enthusiasts to stage a rally failed to attract many supporters and was driven from the streets by anti-Marcos elements.

All unlicensed street protests in the Philippines are forbidden but from the time of Aquino's assassination until today, police had not cracked down unless they were first assaulted.