Thousands of troops patrolled the capital, and some opened fire on demonstrators here today as the government of Gen. Augusto Pinochet sought to suppress nationwide demonstrations called to defy a state of siege.

At the start of a crucial two-day challenge to the government, opposition leaders gathered in the central Plaza de Armas at midday to sing the national anthem and students rallied at campuses around Santiago. Tonight, residents took up what has become the classic protest, banging of pots, and held candlelight vigils while building barricades.

Witnesses said that troops opened fire on demonstrators who tried to build barricades in two southern Santiago neighborhoods. No information was immediately available on injuries.

Ten bombs exploded in Santiago last night, including a powerful car bomb set in a side street facing the presidential palace. In the northern city of Arica, President Pinochet was forced to cancel his schedule of activities after his arrival late yesterday was greeted with violent demonstrations.

The government responded to the opposition with the strongest security measures adopted this year. Army and Air Force troops were stationed on street corners and around bus and train stations, bridges and utilities. Convoys of armored cars filled with soldiers in combat gear patrolled working-class neighborhoods and stopped vehicles for identification.

Police used water cannons to disperse protesters downtown and battled students demonstrating at the University of Chile's medical school this afternoon with tear gas, rubber bullets and buckshot.

Authorities announced that Army reserves would be called up in the coming days to help man increased operations. Troops were used for raids and patrols of Santiago for three days before today's mass deployment. Military mobilizations were also reported in the cities of Valparaiso and Concepcion.

Officials also said they had canceled the credentials of all foreign correspondents working in Chile and that some might not be renewed. Army patrols stopped groups of foreign journalists today and confiscated notes and film.

Government authorities, who have shut down opposition media and censored all news of protests and politics from the internal media, refused to recognize officially that any disturbances were taking place. "The situation of the country is absolutely normal," said Col. Carlos Krumm, a presidential palace official. "Protests? That is not allowed. There is no protest here."

In an apparent effort to maintain a public image of normality, government officials did not alter the existing midnight curfew in Santiago today nor deploy troops in the city's center.

Transport and businesses functioned normally and most of the city remained calm until late in the day. According to local reporters and residents of Arica contacted by news services, violent student demonstrations erupted in two parts of the city as the president arrived yesterday for a 24-hour visit. The city's university was closed.

Troops suppressed the student protests and occupied Arica last night. However, reports said bombs exploded there during the night and parts of the city were blacked out.

Opposition leaders described this week's protest as a crucial confrontation with the government following 18 months of mobilization. "This is a decisive stage," said one top organizer of the Christian Democratic Party who asked not to be named. "If we can defy the state of siege and have a protest, the military strategy of Pinochet will have failed."

In another sense, dissident leaders said they were also fighting for the survival of the movement that seeks to force Pinochet from power and return Chile to democracy after 11 years of authoritarian rule. "If the protest fails, Pinochet will be able to do anything he wants," said the Christian Democratic official, "because it will have been shown that the opposition doesn't have the capacity to respond."

Before attending the midday rally, three top opposition leaders requested and were granted a meeting today with U.S. Ambassador James Theberge. An embassy spokesman said the meeting was attended by Christian Democratic Party President Gabriel Valdes, Socialist Party leader and Democratic Alliance President Ricardo Lagos and Social Democratic leader Mario Sharpe.

Several top opposition leaders said that the media censorship, military mobilization and recent mass arrests around the country were likely to limit demonstrations in Wednesday's informal strike. "What is important is that the opposition has taken the decision to do it," said a leader of the Democratic Alliance. "I don't think the showing itself will be spectacular."

In the last several days, streets in Santiago have been covered with fliers calling for demonstrations ranging from midday assemblies to the banging of pots and pans in homes at night, and opposition sources say 3 million leaflets were distributed clandestinely in the capital alone.

However, protest organizers said their work in urban areas was hampered by the virtual neutralization of leftist parties forced underground by the government crackdown. Communist and militant Socialist leaders in the Popular Democratic Movement have been in hiding since the state of siege was declared and many leftist organizers have moved out of their neighborhoods.