President Amin Gemayel flew to Switzerland today for Lebanon's national reconciliation conference, leaving behind a tense capital deeply worried about a resumption of violence if the talks fail.

The conference, originally scheduled to begin Monday in Geneva, has been postponed until Tuesday, aides said, because Gemayel wanted time to meet privately with the antigovernment opposition, who arrived in Switzerland yesterday.

Gemayel arrived in Switzerland Saturday and told reporters, "I hope this will be beneficial for the world and particularly for peace in Lebanon and lead to the turning of a new page," United Press International reported.

Meanwhile, two U.S. marines were slightly injured today in the first reported assault against them since last Sunday. Spokesman said they were wounded when a rocket-propelled grenade was fired into their position at the Lebanese University science school east of Beirut airport. Marines responded with small-arms fire.

The grim search through the rubble of last Sunday's explosion at the U.S. Marine compound continued today, with the death toll now at 229.

Tonight there was heavy shelling in the mountains overlooking Beirut, scene of battles last month involving Druze fighters against first Christian Phalangist militiamen and then the Army.

From Beirut, the sounds of falling shells could be heard and the flash of rockets seen from rooftops.

State-run radio reported that the artillery duels appeared to be concentrated on Suq al Gharb, a strategic Army-held redoubt on the ridges overlooking the presidential palace and the capital that was the focus of battles before a cease-fire Sept. 26.

The attack on the marines this afternoon followed heavy predawn battles against the Lebanese Army about a mile south of Marine positions.

Later in the morning, guards at the Turkish Embassy here repelled an attack by two gunmen. They captured one who they said admitted he was a member of an underground Armenian group. The gunmen fired bullets across the facade of the building and threw a hand grenade that exploded on a stairway leading to the embassy.

Women and children emptied out of the slums south of Beirut and near Marine positions at the airport this weekend, expressing fear that their communities might be turned into battlefields if the talks in Geneva fail.

A young bank employe who stayed behind bantered with Shiite militiamen at one of their offices in the southern slums this afternoon on the possible outcome of the conference to end the eight years of civil strife.

"Either there's going to be more fighting," he said, pausing for a moment, "or there's going to be more fighting." All laughed.