El Salvador's leftist guerrillas launched their long-anticipated "final offensive" tonight with an attack near the Salvadoran Air Force base in the capital and fitful attempts to mount assualts on scores of townships, barracks and military outposts throughout the country.

The extent of casualties and damage was not immediately ascertainable. But the leftists' declared intention is to push the conflict to a decisive stage after years of increasingly bloody terrorism. An estimated 10,000 people died in political violence here in 1980. The Catholic Church and human rights groups have attributed the majority of the killings to repression by the military and right-wing terrorists.

The revolutionaries are seeking to overthrow an ostensibly reformist civilian-military governmet backed by the United States.

Despite gunfire and fighting in several of the poor sections of the city, much of San Salvador remained calm, its streets largely deserted.

A National Guard commander at general staff headquarters, who declined to let his name be used, said that "this is a general alarm ased on false news. Our armed forces are operating normally. Everyone is in his barracks. It is not members of our forces committing acts of vandalism."

The guerrillas' intentions became known at nightfall when a local radio station -- Radio Romantica -- on the outskirts of the capital was seized by about 20 guerrillas who announced the beginning of the offensive and called for revolutionary support.

"The streets are ours," said one voice. "A free country or death," it added, echoing the slogan of revolutionaries throughout Latin America.

An employee of the radio station who remained in the building said the guerrillas vowed to hold the transmitter for months if necessary, but in fact they left after only an hour.

Before leaving, the spokesman for the group called for an immediate general strike and urged people to join then, to point out government informers and to fight the government wherever its forces were seen.

A leftist spokesman said the guerrillas are attempting to mount actions throughout the country with a plan calling for attacks on more than 50 towns and military outposts.

The heaviest activity is called for in the northern and western areas of the country where the guerillas traditionally have been strong. U.S. military sources estimate there are 5,000 armed, disciplined guerrillas in a population of almost 5 million in this country the size of Massachusetts.

While historically they have been divided in mostly Marxist factions, the guerrillas are now said to be united in the Farabundo Marti National Liberation Front that has declared the "final offensive."

There were some reports of substantial guerrilla columns moving toward the capital this afernoon, but these were unconfirmed, as were accounts of uprisings in several of the poorer neighborhoods of San Salvador.

A government military spokesman estimated that 400 guerrillas who attacked near the Ilopango air base were up against "several hundred" well-trained Salvadoran troops stationed at the Air Force headquarters there. The results of the fight were not immediately clear, but a possible target of the guerrillas may have been at least eight helicopters based at the airport. The helicopters -- many of them French-built Alouettes -- have been used extensively to fight the guerrillas in recent months.

Right-wing paramilitary forces were gearing up for a fight as well. Residents of wealthy residential neighborhoods of the capital were warned that anyone driving in the area would be shot on sight.

The guerrillas, who have worked toward this moment for 10 years and who received at least emotional impetus from the Nicaraguan revolution of 1979, publicly vowed last month to present Ronald Reagan with an "irreversible" military situation here by the time he becomes president of the United States on Jan. 20.

The current Salvadoran government, reshaped several times since young military officers staged a bloodless coup in October 1979, has Washington's backing, and Reagan has said he expects to continue support.

Although military aid to the regime was temporarily suspended amid allegations of the involvement of gvovernment troops in the murder of four American churchwomen last month, Washington reportedly is considering not only renewing but increasing military support, possibly including military advisers.

Those favoring a resummption of military assistance have cited recent U.S. intelligence reports of vast increases in the quantity and quality of armaments reaching the guerrillas.

According to sources in Washington, the leftist guerrilla coalition is now equipped with 50-caliber machine guns, Chinese grenade launchers and 81mm motars. The equipment, according to those sources, has come from Nicaragua, Vietnam, Cuba, Eastern Europe, Libya and the Palestine Liberation Organization.