Police and demonstrators clashed here today, killing at least one student and injuring at least 24 others, in the worst street violence in the capital since opposition leader Benigno Aquino was killed two years ago.

The clash came after about 3,000 protesters, most of them rice farmers, demonstrated peacefully in front of the U.S. Embassy in Manila and then headed toward a downtown plaza. They had planned to march on the presidential palace later in the day to protest government economic policies and U.S. support for President Ferdinand Marcos.

Witnesses said police fired into the crowd as angry demonstrators chanting antigovernment slogans pelted them with stones. Jose Suarez, spokesman for the farmers' group, said at a press conference that police cars cut into the marching column to break it in two.

Witnesses also said four plainclothesmen fired at the demonstrators with shotguns and rifles. One witness, a 14-year-old cigarette vendor, Angelito Garcia, told reporters that he saw five men in green fatigue uniforms come out of a downtown commuter train, aim their rifles at the rear of the column of people below, and shoot them in a short burst of gunfire.

"I saw two men shot in the head," he said. The armed men then went back into the coaches and the train moved on.

The student who was killed was identified as Emmanuel Lazo, 17, according to hospital reports. Manila's western district police chief Brig. Gen. Narciso Cabrera said 12 of the injured were police officers who suffered slight injuries from stones thrown by demonstrators. At least 12 other civilians were hospitalized with serious gunshot wounds, he said.

The official Philippines News Agency said 27 persons were injured.

Cabrera denied that plainclothesmen fired into the crowd. "Witnesses will show that the demonstrators started it," he said. He added that one traffic policeman was manhandled by seven demonstrators while sitting on his motorbike watching the marchers pass. When he sounded an alarm, the riot police surged forward, pandemonium broke out and shots were fired, he said.

Cabrera said the police had followed a policy of maximum tolerance by allowing the marchers to camp overnight at a downtown park. They were among about 10,000 demonstrators, mostly farmers, who had marched into the capital from seven outlying provinces on Sunday. Their main grievances were the low prices of rice and government economic policies they said were sinking them deeper into poverty.

Today's protesters also were demonstrating against the Sept. 20 incident in the Negros Island town of Escalante, where security forces fired on protesting unemployed sugar workers, killing 21.

As protesters retreated from police, they painted street posts and walls with red signs, reading "NPA," the intials of the Communist New People's Army rebels.

Opposition leader Salvador Laurel called today's police action part of a pattern of intimidation by Marcos' "evil regime."