Druze forces today captured the Christian town of Bhamdun near the key Beirut-to-Damascus highway as shells continued to rain down on Beirut, a barrage that left two U.S. Marine members of the international peace-keeping force dead at dawn.

Marine spokesmen said U.S. forces responded only with four 155-mm illumination rounds during the six-hour bombardment around Marine positions adjoining Beirut airport because the fatal barrages had come from a populated area. The two marines killed were identified as Cpl. Pedro J. Valle, 25, of San Juan, Puerto Rico, and Lance Cpl. Randy M. Clark, 19, of Minong, Wis. Two other marines were killed in combat last week.

Two other marines were wounded in the heavy shelling. One, who was taken to the support ship Iwo Jima, apparently was seriously hurt.

The shelling tapered off shortly before midnight.

The barrages came during another bloody day of fighting here that raised fears that the shaky government of President Amin Gemayel could soon fall.

"We are at the point of waiting and seeing, for minutes or for hours," Saeb Salam, leader of the Sunni Moslem community here and a close adviser of Gemayel, said this evening.

Robert C. McFarlane, President Reagan's special Middle East envoy, flew to Damascus late this afternoon to try to get Syria and its Lebanese ally, the Druze Popular Socialist Party militia, to halt the fighting. A Saudi Arabian emissary was already there, and the slender hopes for avoiding a total collapse here rested on their efforts.

Throughout the day, Druze forces bombarded Christian east Beirut and its suburbs, Lebanese Army positions and Christian villages in the mountains from Syrian-held territory and positions in the Chouf Mountains overlooking the capital. The Chouf area was vacated abruptly by Israeli forces on Sunday, touching off full-scale sectarian warfare. The Druze, armed by Syria, are traditional enemies of the Lebanese Christians whose most important armed group is the Phalangist Party's Lebanese Forces.

Marines reported that shells fell near their positions late today as Lebanese Army units near them continued their slow drive to take control of that portion of the coastal highway abandoned by the Israelis in their pullback to a more defensible line in southern Lebanon.

Italian spokesmen said six of their soldiers in the multinational force were wounded in two incidents. None was seriously injured.

Police reported that 216 persons had been killed and 561 wounded since the fighting began Sunday.

Druze forces captured Bhamdun, a Christian Phalangist stronghold on the Beirut-to-Damascus highway in the Chouf, after two days of heavy shelling and ground battles that ended in house-to-house fighting. But the mood of the Druze here in the capital was bitter as they accused the Lebanese Army in two press conferences of abetting Phalangists in a massacre of civilians yesterday in another mountain village.

At the morning press conference for western journalists, Druze spokesmen said 45 people had been killed in a massacre at the village of Kfar Matta. At an afternoon press conference in Arabic, they said more than 100 people had been murdered. In both instances, accounts rested on the report of a farmer who said he had escaped after his wife and two small sons had been shot. He said Phalangists did the shooting but Lebanese Army soldiers stood and watched. The government has assigned Druze Army officers to investigate.

Phalangist Christian radio announcers, in acrid commentaries, conceded the fall of Bhamdun, their former stronghold near the Beirut-to-Damascus highway but they called it a "technical pullback." One Phalangist announcer said that when the Druze entered Bhamdun, they committed "massacres and crimes unprecedented in the history of humanity." Western diplomats here said Phalangists had taken civilians out with them as they retreated. The Phalangists also said they had faced not only Druze militiamen but also Syrians, Libyans and militia of various Palestine Liberation Organization factions in the assault.

Lebanese intelligence sources said Druze and Lebanese leftist allies appeared to be the ones involved in the fighting at Bhamdun but that PLO units were massed behind them.

There were faint hopes yesterday that a victory at Bhamdun would make Druze leader Walid Jumblatt and the Syrians more amenable to negotiations for a cease-fire followed by political agreements. But those hopes were fading today.

"It's not Bhamdun that they want," said Sunni leader Salam, reflecting a widespread anxiety. "They want to finish Lebanon and the regime. If Lebanon falls, not one Arab regime will stand on its legs."

Salam said he had spoken by telephone yesterday with Saudi King Fahd whose emissary, Prince Bandar is in Damascus attempting to arrange a cease-fire. Salam said that Fahd was attempting to stop the fighting and to begin negotiations for a political agreement, but that the Syrians were demanding political concessions first, then a cease-fire.

There were, nevertheless, schemes floated here for appeasing both the Syrians and Jumblatt. One called for a new coalition cabinet that would include Jumblatt, Shiite Moslem leader Nabih Berri and the sons of Christian warlords Camille Chamoun and Suleiman Franjieh.

There was also serious discussion about canceling the May 17 Israeli-Lebanese peace accords, as Syria has demanded.

Patrick Filleux, an Agence France-Presse correspondent who visited the Phalangist Lebanese Forces along the Mediterranean coast Monday night, reported the following:

All night, the Lebanese Forces sent troops, guns and vehicles by sea to strengthen their positions in the Damur region, 12 miles south of Beirut. It appeared that they were anticipating the fall of Bhamdun, about nine miles east of the capital on the Beirut-to-Damascus highway.

Also, in an apparent effort to prevent the Druze from marching southward, the Christians took Kfar Matta, in the mountains about seven miles east of Damur, after more than 24 hours of fierce fighting.

Today, the Lebanese Forces were continuing to reinforce their southern line in preparation for whatever might happen in the Chouf Mountains. They also ordered their gunboats into position as heavy fighting continued Monday night along the coastal road in the vicinity of Khaldah, south of Beirut.

The Lebanese Forces seemed to be relying largely on this sea supply line to bolster their new positions.

Throughout the night, powerful launches laden with men and munitions shuttled between Beirut and Jiye, just south of Damur. The launches sped along without lights, to avoid being seen and shelled by Druze artillery on high positions along the coast.

The commandos, wearing olive green uniforms with the Lebanese cedar tree emblem on their chests, were armed with American M16 rifles, Soviet Kalashnikov submachine guns, West German G3 automatic rifles and RPG rocket launchers.

A small group of men were waiting on the Jiye quayside, some of them wounded in the fighting for Kfar Matta. Crates of 60-mm shells were being unloaded by Indian and Pakistani immigrant workers who formed a human chain from the quayside to heavy trucks.

Once its cargo was loaded, the convoy sped off to Mechref, the site of the Lebanese Forces headquarters for southern Lebanon.

About 40 Druze women and children from the captured village of Kfar Matta were huddled about, fear and fatigue etched on their faces.

They all agreed on the fierceness of the fighting for Kfar Matta, but none of them mentioned the alleged massacre of 45 villagers.