In committing U.S. Marines to another emergency mission in Lebanon, President Reagan yesterday did not place the same limits on their role as when they were first dispatched a month ago.

The stay of the earlier force was limited to 30 days, but no such deadline has yet been set this time, according to senior officials. A longer stay could require the approval of Congress, which reacted to the president's announcement with a mixture of nervous approval and doubt yesterday.

The force of about 800 men will be about the same size and carry similar weapons as the earlier unit that participated in a multinational force overseeing the withdrawal of Israeli-surrounded Palestinians from West Beirut, according to senior administration officials. The Marines could be back on the ground in Beirut in 72 hours, but final deployment is still being negotiated and Israel has not yet agreed to it.

"The international community has an obligation to assist the government of Lebanon in reasserting authority over all its territory," Reagan said in a 10-minute television address in which he announced that the multinational force made up of French, Italian and American troops would return to the Lebanese capital following the weekend massacre of hundreds of Palestinians in refugee camps in West Beirut. French President Francois Mitterrand and the Italian government also announced their participation yesterday.

Reagan described the mission of the force as "enabling the Lebanese government to resume full sovereignty over its capital and the essential precondition for extending its control over the entire country."

Reagan and other U.S. officials stressed that the multinational force would not have the responsibility of policing the violence-ridden Lebanese capital. They said the role of the force would be "to make it possible for the lawful authorities of Lebanon to do so for themselves." But they did not specify how this would be done.

"The force will take up positions that are important," said a senior official, "because they mark a line between two populations that are hostile. Their very presence will provide a measure of stability and international attention." He added that "undoubtedly it will be a factor in the withdrawal of Israeli forces."

The government of Lebanon will have the "basic task" of internal security, the official said. "Actual deployment of the multinational troops is in the process of being worked out. It will be along the lines of deployment before, maybe a little farther south."

The earlier force consisted of 800 U.S. Marines, deployed mainly in the port area of West Beirut, and 800 French Foreign Legionnaires and 530 Italian troops, who were deployed at major crossing points along the "green line" that separates the Christian and Moslem parts of the Lebanese capital. West Beirut is predominantly Moslem.

The official's reference to deployment "a little farther south" appeared to refer to the Palestinian camps where this weekend's massacre occurred. The senior official said pointedly that troops from the multinational force would not enter the camps themselves.

"If the government of Israel says they will oppose the multinational force, we won't deploy," the official said, "but I don't think that will be the case."

The official said Secretary of State George P. Shultz had been in touch with Israeli Foreign Minister Yitzhak Shamir about the force.

"We haven't received any big negative in exchanges with Foreign Minister Shamir," the official said, adding that Shamir had made a point of noting that Israeli forces were in the process of turning over positions in West Beirut to the Lebanese Army.

In his television address, President Reagan said "millions of us have seen pictures of the Palestinian victims" of this weekend's massacres in the camps of Beirut. "There is little that words can add."

In an apparent appeal for support from the American public for this new use of U.S. troops abroad, Reagan said: "It is not enough for us to view this as some remote event in which we ourselves are not involved. For our friends in Lebanon and Israel; for our friends in Europe and elsewhere in the Middle East, and for us as Americans--this tragedy, horrible as it is, reminds us of the absolute imperative of bringing peace to that troubled country and region."

He said special negotiator Morris Draper would continue his efforts to get all foreign troops out of Lebanon and would be joined soon by the man who negotiated the terms for the initial multinational force, Ambassador Philip C. Habib.

Habib participated in the special White House meeting yesterday morning when the decision to re-establish the multinational force was made. Reagan said Habib would return to Lebanon for the inauguration of a new president when a successor to president-elect Bashir Gemayel is chosen.

It was the assassination of Gemayel last Tuesday that triggered the Israeli military movement into West Beirut with the stated aim of keeping hostile Moslems and Christians separated.

The focus of yesterday's U.S. moves was on the volatile situation in West Beirut, but Reagan emphasized several times in his remarks the need for withdrawal of all foreign forces from Lebanon and the importance in his view of that step for overall peace in the Middle East.

He said withdrawal of Israeli troops from Beirut was "essential" for the multinational force to succeed. He warned the Israelis more than once that they could be dragged deeper and deeper into Lebanon's tangled politics should they linger there.

At one point, the president referred to the "quagmire" of Lebanon and at another he said, "Unless Israel moves quickly and courageously to withdraw, it will find itself ever more deeply involved in problems that are not its own and which it cannot solve."

Apparently anticipating criticism of deployment of U.S. troops on the same grounds, Reagan added, "The participation of American forces in Beirut will again be for a limited period. But I have concluded that there is no alternative to their returning to Lebanon if that country is to have a chance of standing on its own feet."

Reagan again said, as he had in remarks to a political campaign audience last Friday, that the withdrawal of foreign forces from Lebanon must come "very soon." The State Department and a United Nations resolution which the United States supported call for "immediate" withdrawal.

The president said that both Israel and Syria had indicated a willingness to pull their forces out of Lebanon. "The legitimate security concerns of neighboring states, including particularly the safety of Israel's northern population, must be provided for," Reagan added, "but this is not a difficult task if the political will is there."

Both Reagan and the senior administration official who briefed reporters at the State Department also referred to the necessity a meeting the "legitimate rights of the Palestinians."

When it became clear over the weekend that international action might be required in Beirut following reports of the killings in the Palestinian camps, attention first focused on the United Nations.

A U.N. resolution called for redeploying to Beirut of United Nations forces already based in southern Lebanon. The senior U.S. official said that U.N. Secretary General Javier Perez de Cuellar had asked Secretary of State Shultz to sound out the Israelis.

The official said Foreign Minister Shamir responded that the Israeli Cabinet found this "troublesome." But the Israeli response was made moot when the Lebanese government "expressed a preference" for the multinational force.

The top-level decision team that met over the weekend and yesterday morning at the White House is understood to have considered "all the options" but to have given serious consideration only to the multinational force, over which participating governments would have greater control.

If the reconstituted multinational force reaches Beirut within 72 hours as officials indicated yesterday, it would arrive at just about the time the original force had been expected to end its mission.

A State Department timetable for the original force had placed the pullout between Sept. 21-26, following a two-week period in which it was to have assisted the Lebanese Army in reaching "arrangements, as may be agreed between governments concerned, to ensure good and lasting security throughout the area of operation."

The U.S. Marines in the initial contingent pulled out Sept. 10 within days of the departure of Palestinian and Syrian forces from Beirut, and the rest of the multinational contingent left over the next week to 10 days. The Marines were in Lebanon 16 days.

In assessing the role of the first force, Reagan said yesterday:

"Early in the summer, our government met its responsibility to help resolve a severe crisis and to relieve the Lebanese people of a crushing burden. We succeeded. Recent events have produced new problems and we must again assume our responsibility."