SANTIAGO, Chile (AP) _ Army troops fired on rock-throwing demonstrators in a Santiago slum during a nationwide general strike Wednesday, and a 2-year-old boy was killed by a stray bullet, authorities and radio reports said.

Dozens of other clashes were quelled by tear gas, water cannon and police with nightsticks.

About 400 people, including six prominent Socialists, were detained by police nationwide, according to police sources and human rights organizations. At least ten people, three of them a policeman, were wounded in daylong protests in Santiago.

It was the biggest single day of protest against the right-wing military government of President Augusto Pinochet since a two-day strike in July 1986, when eight people were killed.

Less violence was evident Wednesday, despite political tension fueled by the maiming of a police officer Tuesday in a terrorist bombing, the 37-day-old kidnap of an army colonel, and the reported disappearance of five members of the outlawed Communist Party.

Shortly before midnight Wednesday, radio reports and residents of the Penalolen neighborhood in southeast Santiago said a 2-year-old boy, Felipe Gutierrez, was killed. He was hit by a stray bullet while sleeping in a municipal shelter, they said. The reports said shots were fired during disorder in the area. Police did not immediately confirm the death.

In La Victoria, a slum on the city's southern outskirts where anti- government sentiment runs high, three protesters suffered minor injuriess from buckshot fired by soldiers, witnesses said. The soldiers were retreating from a group of youths tossing stones, the witnesses told reporters.

In another incident in La Victoria, reporters saw national police fire four shots at a group of rock-throwing demonstrators, but no one was hit.

The protesters mounted barricades of burning tires on several streets.

In downtown Santiago, bands of youths skirmished for hours with police in riot gear, who used water cannon, tear gas and nightsticks to break up the protests. The demonstrators would scatter and then quickly regroup, chanting anti-Pinochet slogans such as ''He is going to fall 3/8''

In a clash along Santiago's main throughfare, two demonstrastors were wounded by buckshot fired by riot police, witnesses said. A police officer was injured in the face.

The skirmishes sent crowds of bystanders, many dressed in business suits, fleeing through the streets with handkerchiefs pressed to their faces to ward off the effects of the gas.

In the central Plaza de Armas, police detained and later released six prominent Socialist Party leaders who were singing the national anthem from the steps of the city cathedral. They included party leaders Ricardo Nunez and Jorge Molina.

A spokesman for Chile's independent Human Rights Commission quoted a police officer as reporting that 200 to 250 people had been detained in Santiago.

Arrests were also reported in some provincial cities. In Valparaiso, Chile's second largest city 75 miles northwest of Santiago, police said 70 demonstratros were detained.

A bomb damaged a railroad line near Valparaiso, the country's principal port 75 miles northwest of Santiago. No one was injured, police said.

The day-long strike was called by the National Workers' Command, the main trade union federation, to press the Pinochet government for a 30 percent wage increase and a minimum monthly wage of about $85. The minimum wage is about $50.

It drew only partial support. In the Santiago area, where 4.5 million of the country's 12 million people live, most shops were open for business, although few public buses were on the streets and pedestrian and car traffic was lighter than normal. Even less impact was reported in provincial cities.

As skirmishes continued into midafternoon, however, many shops closed, public transportation virtually disappeared, and Santiago's normally crowded downtown area emptied.

A command spokesman said participation in factories in the industrial belt around Santiago ranged from 30 percent to 80 percent. However, he said that all steelworkers nationwide had respected the strike call.

Interior Minister Sergio Fernandez, after meeting with Pinochet and other Cabinet members, called the strike ''a complete and utter failure.'' He said the government was considering legal action against union leaders.

The command's president, Manuel Bustos, had appealed Tuesday night for protesters to avoid violence after a terrorist bombing injured two policemen in Santiago. One of the officers lost an eye, his right hand and the fingers on his left hand when the bomb went off outside a bank. He was reported in serious condition Wednesday.

The protest also coincided with the continuing search for an army colonel, Carlos Carreno, kidnapped outside his home Sept. 1 by the Manuel Rodriguez Patriotic Front, a pro-communist urban guerrilla group. Carreno's whereabouts remain unknown and news media have been barred from reporting on the case.

In the days following the kidnap, five Communist Party members disappeared, according to human rights groups. The Communist Party alleges they were abducted by the secret police and are being held hostage pending Carreno's release. The government denies it is holding them.