The 800 U.S. Marines of the Beirut peace-keeping force withdrew today as Moslem leaders expressed concern about what will happen in West Beirut after the entire international force pulls out and leaves Israeli troops in positions around the city.

Sixteen days after their arrival here as part of the 2,130-man, three-nation force to supervise the evacuation of Palestinian guerrillas -- and 14 days ahead of schedule -- the men of the 32d U.S. Marine Unit reboarded their ships to sail off into the Mediterranean Sea for what their commander, Col. James Mead, called "some well-deserved liberty" in Europe.

As an Israeli reconnaissance jet roared overhead and a shuttle of helicopters ferried two companies to the assault carrier USS Guam anchored a mile away, Mead proclaimed: "We were given a job to do here, and we did it. Now it is time to go, and we are going."

The Marines' departure heralded the beginning of the end of the international force set up to guarantee the Palestinian evacuation and the normalization of Israeli-besieged West Beirut under an accord negotiated by U.S. special envoy Philip C. Habib. The 530-man Italian contingent will leave Saturday, while the 800 French Foreign Legionnaires and paratroopers will withdraw by the end of next week.

The winding down of the international force worried Moslem leaders who had expected the Israelis to pull back from their positions around the city before the final withdrawal of the international force. With the Israelis still at the city's gates, Prime Minister Shafiq Wazzan asked early this week that the international force stay on, at least for the full 30 days originally envisaged by the Habib plan.

U.S. officials rejected the request, maintaining that, with the evacuation of the Palestine Liberation Organization completed, the multinational force's primary role had been accomplished even though Israel had not withdrawn as it had been suggested it would. With the United States refusing to keep its Marines in Lebanon, both Italy and France decided they would leave since they considered U.S. participation in the force a guarantee of Israeli acceptance of its role.

"It was not my decision that we leave," said Mead as he watched troops board the helicopters. "Our mandate was to come in for 30 days or less. The nations involved in the force agreed it is time to withdraw, and I'm following those orders."

The Marines' mission in Lebanon was rigidly limited by U.S. concern about their vulnerability in this volatile city of heavily armed militias, both Christian and Moslem.

Just how many imponderables remain in Lebanon as the Marines departed was underlined by the group of Israeli soldiers who watched beside two Merkava tanks parked next to a port warehouse in violation of the Habib accords that had specified that the Israelis were to have withdrawn from the port area when the multinational forces arrived.

The Israeli tank crews were part of a group of Israelis who balked at leaving the port and stayed on to observe the PLO evacuation. The multinational force commanders eventually decided not to insist that they leave.

A Lebanese Army colonel, who will assume charge of the port when the last Italian and French forces leave, simply shrugged when asked if he was not worried about the continuing presence of the two Israeli tanks.

"What are two tanks?" asked the colonel, refusing to give his name. "In Lebanon today the Israelis are everywhere."

The still unresolved issue of an Israeli withdrawal, has been left to a second phase of negotiations to be led by Habib's former deputy here, U.S. Assistant Secretary of State for Near Eastern and South Asian Affairs Morris Draper. The Israelis have said they will not withdraw until PLO and Syrian forces in northern Lebanon and the eastern Bekaa Valley also withdraw.