Air Force jets strafed downtown La Paz today in an effort to break up demonstrations against the newly installed military government of Col. Alberto Natusch Busch. Continuing clashes during the past 24 hours left another six persons dead and 21 injured.

Natusch, who led a military coup Thursday that overthrew this country's first nonmilitary president in 15 years, enforced a series of increasingly harsh measures in an effort to stamp out stubborn resistance to his leadership. h

Natusch announced today that he was imposing martial law, which means that any person found armed or disturbing the peace may be shot on sight. He also announced that the state of siege imposed last Thursday would be strictly enforced, that a 10 p.m. curfew would take effect this evening and that press censorship would go into effect.

None of La Paz's three Sunday newpapers was published today, and most of the city's 22 radio stations remained off the air. The government announced that all schools and the University of San Andres would be closed for another week, saying that the university had been "infiltrated by extremist elements."

All foreigners were required today toobtain identity cards from the military police at the National Directorate of Investigation.

During the night, artillery units gutted the headquarters of Bolivia's central labor organization, which is responsible for the continuing and effective general strike that has paralyzed La Paz since Natusch assumed the presidency.

Throughout the day and into the evening, machine-gun fire, rocket exposions and small-arms fire continually punctured the silence of the city's otherwise still and almost deserted central business district.

Natusch came to power promising to "strengthen democracy" in Bolivia and respect the individual and political rights of the country's 5 million people, but he changed course today, branding union leaders "professional agitators," and calling student and worker opponents of his rule "subversives" orchestrated by a "brigade of terrorist mercenaries."

Bolivia's military is known to be badly divided, and Navy units stationed near La Paz reportedly began a short-lived rebellion against Natusch this morning. But diplomatic sources said country's top military leaders decided in favor of Natusch yesterday, giving him their blessing to impose a harsh military dictatorship if necessary to quash oppostion to his unpopular and politically isolated government. The sources said the military leaders backed Natusch to preserve the unitary of the armed forces. Their decision was made despite efforts by at least two mediators seeking Natusch's resignation to avoid further bloodshed and a possible civil war.

Natusch moved quickly to end the dissent. Informed sources here said that the two most important military, opponents of Natusch's government, Lt. Col. Raul Lopez Leyton and Col. Gary Prado, were arrested yesterday evening and were being held in the Ministry of the Interior.

At 12:30 a.m. today, armed soldiers detained leading members of the National Revolutionary Movement, a moderate political coalition headed by Victor Paz Estenssoro, a powerful political figure who has served as president several times.

Among those arrested and held for almost 10 hours were three senators and 12 deputies -- all of them elected in July to Bolivia's now suspended Congress. One of those arrested said that the 15 congressmen were kept for more than three hours in a dark room at the Interior Ministry and forced to stand with their hands against a wall until finally being allowed to rest.

Before their release, the 15 were told by an Interior Ministry official that they ought to cooperate with the new government because both Natusch and the military plan to remain in power indefinitely.

Meanwhile, Walter Guevara Arze, the country's deposed civilian president, remained in hiding for the fourth consecutive day. In addition, Paz Estenssoro and Juan Lechin, the leader of the central workers' confederation also reportedly had gone underground because of fears that they will be arrested and possibly deported from Bolivia if found by the military.

The now government issued several communiques throughout the day, warning workers that they are expected to report their jobs Monday in an obvious attempt to break the four-day-old general strike. The government also said it would impose corporal punishment on anyone found hoarding or speculating on food prices.

Many observers here believe that Monday will be a crucial day for the new government gecause the general strike is seriously affecting supplies of meat, fruit, vegetables, gasoline, c igarettes and other consumer goods in this city. Cash is in short supply.

Transport is nonexistent because taxi, bus and truck drivers have stayed at home. Banks have been closed since Wednesday and the city's airport has been virtually closed since Thursday because controllers belonging to the workers' confederation have not reported for work.

It is generally expected that the government will have to break the strike by force on Monday or, at the latest Tuesday, if its warning to workers to return to their jobs is widely ignored.

Throughout the day, isolated groups of students and workers built barricades and reportedly shot at troops in some parts of La Paz. But, for the most part, the demonstrators were unarmed, according to eyewitnesses who saw troops begin firing on several occasions to disperse crowds.

The jets strafed La Paz's main avenue shortly before noon to break up a crowd gathering in front of the labor headquarters building, destroyed during the night.

"The more they shoot at them, the more likely the situation will explode," said one civilian politician today. "the people here are slow to react, but when they do, they become very violent and very difficult to stop."