At least 18 persons were reported killed today as police opened fire during three separate demonstrations in the center of the city by members of a militant student-peasant coalition.

Members of the Popular Revolutionary Bloc, the same coalition that took over the Costa Rican and French embassies here Friday, kept returning to the main square between the cathedral and the national palace despite police warnings.

By late afternoon, when police cleared the downtown area of both demonstrators and workers, bodies lay covered with sheets in the square.

Earlier today, the demonstrators took over a radio station and forced the disc jockey on duty to read a 15-minute manifesto repeating their demands that the government release five political prisoners, including the Bloc's secreatary general.

Other members of the Bloc continued to hold five persons hostage, including Ambassador Michel Dondenne and Counselor Jean Duffaud, in the French Embassy. Costa Rican Ambassador Julio Esquivel Valverde and four other employes escaped from the Costa Rican Embassy Monday night while their captors were eating.

Speaking to reporters about today's shootings, a national police spokesman said members of the Revolutionary Bloc who also took over the Metropolitan Cathedral Friday fired first, critically wounding three policemen. The spokesman, Col. Rafeal Flores Lima, said the government may impose a form of martial law if the violence continues.

Earlier, a government spokesman said El Salvador "has never negotitated with any subversive group and will not" do so in this case.

By late today there were widely conflicting accounts on the number of casualities. The Associated Press reported that the Red Cross confirmed at least 18 demonstrators were killed and at least 37 were wounded. Police gave no casualty figures for the demonstrators. A police spokesman said 30 officers were wounded.

The Bloc is a militant organization of leftist students and farm workers demanding that the military government of Gen. Carlos Humberto Romero institute social reforms and investigate alleged human rights violations in this poor, densly populated Central American country that long has been dominated by a small, wealthy oligarchy.

Today's demonstrations started when more Bloc members went to the cathedral. Members of 15 important trade unions, including construction and brewery workers and the teachers, staged simultaneous 1-hour work stoppages.

The demonstrations escalated as about 300 persons marched on the central square, where the cathedral is.

Around noon, steel-helmeted national police surrounded the area and ordered the demonstrators to leave. When they refused, police began firing G3 automatic assault rifles and submachine guns into the air. When about 100 demonstators broke from the crowd and ran for the cathedral, however, the police "lowered their aim," a witness told the Associated Press.

Some of the demonstrators were reported killed as they scattered into side streets in the downtown area.

Witnesses reported dozens of casualities on the steps of the centuries-old cathedral and a television crew reported seeing two police officers lying wounded in the street.

About 30 minutes later, the demonstrators regrouped and marched once again on the cathedral. Police fired again, killing more persons, witnesses said.

An hour later, the demonstrators gathered again to meet police fire. This time more persons were reported killed.

In was unclear by late afternoon whether all 18 of those killed were Bloc members or whether some were bystanders or police.

By late afternoon, the police managed to quell the demonstrations and clear the crowded downtown area.