The Reagan administration yesterday hailed Israel's acceptance of the plan for the evacuation of Palestinian fighters from West Beirut, but it temporarily postponed the announcement of U.S. participation.

In preparation for the role assigned to it under the evacuation plan, worked out with the aid of U.S. emissary Philip C. Habib, a U.S. Marine amphibious force has moved into position in the eastern Mediterranean within reach of the Lebanese capital, the Pentagon said.

Aboard the five ships are 1,800 Marines who left Naples, Italy, for the eastern Mediterranean on Monday. Of these, 800 Marines are expected to participate, along with 800 French troops and 400 Italian troops, in monitoring the departure of Palestine Liberation Organization guerrillas.

President Reagan announced his agreement in principle July 6 to participate in the operation under certain conditions, including agreement of all the parties to the conflict in Lebanon.

Several senior members of Congress expressed reservations about the plan but, after briefings at the White House and on Capitol Hill, it was apparent that Congress would go along if, as Reagan claimed, the U.S. military participation was essential to a peaceful resolution of the struggle in Beirut.

The formal request from the Lebanese government for U.S. participation was received in a cable at 7:45 a.m. yesterday, according to White House deputy press secretary Larry Speakes.

At about the same time, the Israeli Cabinet approved the plan, which Habib had sent to Israel Wednesday.

A written White House statement said, "We are, of course, extremely gratified by the Israeli Cabinet's approval of the plan, which is a tribute to the remarkable diplomatic achievement of the president's personal emissary, Amb. Philip Habib."

The statement added, "It sets the stage for implementation of the plan, which we expect to start as early as this weekend."

The White House did not announce immediate approval of U.S. participation, saying unspecified "remaining matters" must be cleared up. There seemed no doubt that Reagan will approve the plan, largely engineered by Habib despite a host of difficulties in the area.

Officials led reporters to expect a formal announcement by Reagan this morning, followed by a news conference by Secretary of State George P. Shultz. It would be his first news conference since assuming office five weeks ago today.

There was no clear-cut statement of the nature of the "loose ends" said to have caused the temporary delay. Among the factors, according to official sources, were:

* A delay of several hours in obtaining final approval in the Middle East of details of the PLO's return of two captured Israeli soldiers.

* A belief attributed to Habib that all understandings and implementation details should be approved one last time by all parties, to avoid any possible complications later.

* A flurry of concern about statements that were attributed to Syrian sources and seemed to cast doubt on evacuation movements of the Syrian-controlled Palestine Liberation Army, one of the Palestinian military groups in Beirut.

* The intense White House concentration on yesterday's House showdown over the tax bill. Some officials said they expect the administration to confer again with congressional leaders, at least on a cursory basis, before announcing deployment of the U.S. troops in Lebanon.

The White House announced previously that, in compliance with the War Powers Act, it would tell Congress about dispatch of Marines to Lebanon. It was still uncertain, however, whether the report would be made under a provision limiting deployment to 60 days, or under a less stringent provision with no time limit.

Sources here said the U.S. troops are not expected to be sent ashore in the initial stage of the Palestinian withdrawal, which could start as early as Saturday. These sources said the French forces will go first, with the U.S. Marines expected to land in Beirut next Wednesday or Thursday.