Spearheaded by tanks and backed by intense artillery barrages, Vietnamese troops today opened their long-awaited assault on the headquarters of the main noncommunist resistance group battling Vietnam's occupation of Cambodia.

By Monday night, about three-quarters of the Khmer People's National Liberation Front base of Ampil was in Vietnamese hands, The Associated Press reported, quoting Thai military sources. They said that the guerrilla defenders split into small groups and slipped away.

The Vietnamese attack on the camp, located just across the Cambodian border from this Thai village, coincided with the sixth anniversary of Vietnam's capture of the Cambodian capital, Phnom Penh, after a lightning invasion in December 1978.

From the din of battle that could be heard here, it appeared that the Vietnamese met some stiff resistance. The fighting, which was reported to continue late into the night, was shaping up as the most crucial of the current dry-season campaign along the Thai-Cambodian border.The battle involved several thousand troops on each side.

A French correspondent and photographer were in the camp as the attack began. Details, Page A11.

The commitment of troops, armor and artillery was far bigger than that made before an abortive Vietnamese attempt to overrun the base last April, which bogged down amid reportedly heavy casualties. Western analysts said today's assault would not have been launched had Hanoi not felt confident of a clear-cut victory at Ampil.

Vietnamese troops already have overrun a string of Khmer front bases and raised a major question mark over the political and military future of the resistance group. The Khmer front and a smaller noncommunist faction led by Prince Norodom Sihanouk, the former chief of state in Cambodia, are partners in an uneasy coalition with the communist Khmer Rouge. By far the strongest of the resistance groups, the Khmer Rouge were driven from power in Phnom Penh after ruling brutally for nearly four years.

Today's battle opened at 6:30 a.m. with sustained fire from an array of Vietnamese artillery.

As artillery fire continued to rain in on Khmer front positions, Vietnamese infantry units spearheaded by Soviet-built T54 tanks launched a three-pronged attempt to punch through Ampil's crescent-shaped defense perimeter. The camp, set on flat, sparsely wooded terrain, backs directly onto an extended antitank ditch that for military purposes marks the Thai border.

Thai military sources monitoring the battle said Vietnamese forces lost three tanks to either mines or rocket-propelled grenades.

The AP quoted the commander of Thailand's 1st Army Region as saying later that seven Vietnamese armored vehicles had been destroyed by the retreating rebels.

At least 20 guerrillas were killed and parts of the base were destroyed, said intelligence sources in the Thai Eastern Force. One rebel leader said 100 Vietnamese were killed or wounded, but the figures could not be verified independently.

By 8 a.m., Thai villagers from the hamlets of Ban Sangae were being hastily evacuated by Thai troops and police as Thai armored vehicles moved into position along the antitank ditch behind Ampil. Trucks towing heavy artillery and loaded with troops also were seen speeding from Thai Army camps six miles away, toward positions opposite the embattled Khmer front base.

During the morning, Thai artillery was heard returning Vietnamese fire that overshot Ampil and landed on Thai soil. The commander of the Thai Army's Eastern Task Force, Maj. Gen. Salya Sripen, told reporters that up to 20 Vietnamese shells had landed in Thailand, wounding four Thai rangers.

Diplomatic sources in Bangkok said the extent to which Thai artillery becomes involved in the border battle may prove crucial in deciding Ampil's fate. "The artillery is going to be a big factor," said one senior western diplomat before the battle began. "It could be decisive."

Meanwhile, indications are that the Khmer front, which has anticipated the assault since the fall of their largest camp at Rithisen to the southwest on Christmas Day, is continuing to put up stiff resistance.

The Khmer front is estimated to have committed at least 4,000 of a total of about 15,000 guerrillas to the defense of Ampil.

Before the assault, Khmer front troops in camouflage fatigues were seen preparing well-dug positions for new Chinese 12.7-mm antiaircraft guns.

Pitted against Ampil's defenders in the current fighting are about 5,000 to 6,000 troops from three infantry regiments of Hanoi's 5th Division and elements of a fourth regiment. They are backed by 105-mm, 122-mm, 130-mm and 155-mm howitzers that opened this morning's fighting. The Vietnamese also have brought up at least 12 aging T54 tanks and 20 former U.S. M113 armored personnel carriers, western diplomats said.

Analysts said the symbolic importance of Ampil, heightened by the recent fall of five of the Khmer front's eight camps along the Thai-Cambodian border, has raised the stakes here dramatically. The front has only two remaining bases not hit in the current offensive -- a small camp near Ampil at Sanro Changan housing about 10,000 refugees and some guerrillas and a civilian camp of 20,000 refugees at Dongrek, about 20 miles away.