A large force of leftist guerrillas attacked the Salvadoran government's garrison at the major provincial capital of Usulutan shortly before dawn today and sustained the assault for eight hours before pulling back, witnesses said.

It was the guerrillas' first such daylight assault on a city of this size. Usulutan, with about 25,000 population, is the fourth largest center in the nation of 4.7 million.

This attack, and assaults on several smaller towns to the northeast begun yesterday, apparently are part of a significant increase in activity promised by the insurgents in recent propaganda and public statements. The attacks may also be intended to test the response capability of the Salvadoran Army since five of its vital 14 U.S.-supplied helicopters, as well as at least 10 other transport aircraft, were blown up in a guerrilla raid last week.

In Washington yesterday, the Reagan administration announced that it is sending $55 million in emergency military aid to El Salvador. Assistant Secretary of State Thomas O. Enders told congressmen that to withhold aid now would mean "probable victory" by the guerrillas.

There was no immediate military comment on the Usulutan fighting nor an estimate of casualties. Witnesses who traveled there, about 75 miles to the east, did report seeing one dead policeman and three wounded soldiers. The number of guerrilla attackers was not known, although the intensity of the assault, made with automatic rifles and rocket-propelled grenades, indicated a force of substantial size.

Following the Jan. 27 raid on the helicopters, only three of the craft are believed to be operational. None was seen in the area of Usulutan, the witnesses said. Late this afternoon at least one UH1H (Huey) helicopter was observed landing in the capital at the field frequently used to receive wounded soldiers.

The guerrillas fighting to overthrow the U.S.-backed military-civilian junta here announced over their clandestine Radio Venceremos last night that they would step up their military activity "very soon" both here in the capital and elsewhere.

The broadcast warned residents of the poor neighborhoods of San Marcos, Soyopango, Cuscatancingo and Mejicanos, on the edges of the city, to be ready. Medical students were told to be prepared to help squads of militia act as backup for guerrilla regulars.

Leaders of the guerrilla coalition known as the Farabundo Marti National Liberation Front have said repeatedly in recent months that they would not attempt to launch another offensive on the scale of the "final" one that failed a year ago when coordinated attacks were launched against garrisons in virtually all parts of the country. At that time, before the United States began sending "lethal" military assistance to the government, not one garrison fell.

This time the guerrillas have said they will steadily increase their pressure on the Army, especially with the approach of elections scheduled for March 28 for a constituent assembly.

Government counteroffensives --the most recent one in December in Morazan Province--have made little headway against the insurgents. When the Morazan offensive ended, the guerrillas immediately started knocking out military emplacements one by one in various small villages to give themselves ever freer access on the approaches to major towns.

Yesterday, such attacks occurred at the Morazan village of Corinto and Nueva Trinidad in the department of Chalatenango. Corinto reportedly is still in guerrilla hands, and several members of government uniformed and paramilitary forces are reported dead in both attacks.

Reuter quoted informed military sources in San Salvador as saying about 100 persons were killed in Nueva Trinidad, including the local military chief and 11 soldiers.

In Usulutan, to the southeast of the capital, the city center was shut down tight at midday and appeared almost deserted, said news photographers able to make their way close to the fighting. Most of the shooting was from fixed, well-protected positions, and for that reason the total number of casualties was believed to be low.

The guerrillas apparently had taken up numerous positions enabling them to sustain an intermittent but intense fusillade against the garrison.

A small airplane circled above the action as guerrillas fired at it regularly. Occasional high-powered explosions near the guerrilla positions were attributed by some witnesses to bombs, but the airplane appeared to be an observation craft, they said.