U.S. Marines landed here today, the second time in less than a month, as the Israeli Army withdrew its last troops from the international airport and the Christian eastern sector.

With all Israeli troops removed from the Lebanese capital for the first time since June 13, the vanguard of the expected 1,200 Marines arrived by ship at the port and by helicopters from the aircraft carrier Guam off shore. The troops of the 32nd Marine amphibious unit began setting up camp inside the grounds of the airport, which they have been assigned to guard alongside the Lebanese Army.

The Marines are joining 2,200 French and Italian troops rushed here following the massacre of hundreds of Palestinians in refugee camps in the southern districts of West Beirut Sept. 16-18.

The peace-keeping force was withdrawn in mid-September after the Reagan administration decided to pull out the U.S. contingent following the evacuation of guerrilla and Syrian forces from Beirut. President Reagan said yesterday that this time the Marines will stay until the departure from Lebanon of all other foreign forces, apparently meaning those of Israel and Syria.

In Washington, a State Department spokesman said Wednesday that the president "was not setting a criterion" for the Marines' departure. He said that it would be up to the Lebanese government to ask the Marines to leave.

In a letter to Congress, Reagan said the Marines' stay would be "limited," but he left the legislative branch no role in saying just how long. Story, Page A36.

Col. James Mead, commander of the Marines, arrived at the airport just before the last Israeli soldiers pulled out of their base there and drove south in jeeps. He said he was returning with "very mixed emotions" because of the causes that had prompted the redeployment -- the murder of Lebanon's president-elect, Bashir Gemayel, and the massacre.

Mead led the same Marine unit when it came to West Beirut in late August as part of the multinational force put together to oversee the evacuation of Palestinian guerrillas.

The then 800-man Marine force was confined to the port and was scarcely seen by the public. This time, the Marines are to be highly visible at the international airport.

In the past five days, the airport has taken on symbolic importance as U.S. and Israeli authorities bargained over whether Israel would be allowed to maintain access to it.

The issue involved not only U.S. prestige and a test of will with Israel but the question of restoring Lebanese sovereignty over the capital area. It delayed the arrival of the Marines by at least three days because the Reagan administration refused to have them come ashore until the Israelis left all of Beirut.

U.S. Ambassador Robert Dillon, who was on hand to greet Col. Mead and the Marines together with U.S. special envoy Morris Draper, said there were two basic reasons why the United States had made such an issue over the airport. First, he said, the departure of the Israelis was "simply a very important symbol" of the return of Lebanese sovereignty.

"We think that Beirut as the capital of the country has got to be free and unambiguously under the control of the Lebanese government," he said.

The second reason he cited was that the U.S. officers had wanted to have "a clean operation," apparently meaning they did not want to land and get into a feud with the Israelis over who controlled what at the airport.

The final terms of the agreement over the airport, according to Dillon and Israeli military spokesmen here, allow the Israelis to use it only in "emergency cases" and with the prior approval of Lebanese authorities. The first flights from the airport are to go to Europe Thursday.

Still unresolved was whether Israel was also about to evacuate Baabda, the site of the Lebanese presidential residence and often considered the country's seat of government.

An Israeli Army spokesman said on Tuesday, "Baabda is not Beirut. Baabda is Baabda," and indicated the Army was staying on there. The suburb is located about three miles east of Beirut and is also the site of the American ambassador's residence.

Asked whether he agreed with the Israeli spokesman's statement, Dillon replied: "Baabda may not be Beirut. But it is the seat of the Lebanese government. It is clearly just as important to see it free of Israeli troops as Beirut proper."

This seemed to indicate the next issue in the ongoing tug-of-war between the Israeli and U.S. governments over the Israeli presence here would be Baabda. Today, the Israeli military spokesman at Baabda was no longer speaking confidently of staying on there. Lt. Col. Yoccob Perez, the deputy Army spokesman at the Israeli press center there, said it was no longer certain the center would remain open.

The Israelis had been hoping to leave behind a residual military and civilian presence, apparently to establish a basis for a de facto normalization of relations with Lebanon. Today's pullback leaves Israel's closest units at the Khalde junction just below the airport perimeter. Thus, they still control all access roads to the capital from the south and east.

The area marked out for the Marines is largely unpopulated, though it is not far from the Palestinian camp of Burj al Barajinah.

"The U.S. Marines are not involved in any way, shape or form with the Palestinian camps," said Lt. Col. Lee T. DeLorme Jr., a public affairs official from the secretary of defense's office.

French and Italian troops have already taken positions in camps where the massacres occurred.