Nearly a week after the announcement that U.S. Marines may go into Lebanon, there is still controversy within the government about the roles and the risks they would assume.

According to the White House announcement last Tuesday, the specific missions of the multinational force, including 800 to 1,000 U.S. Marines, "would be to assist Lebanese armed forces in the orderly and safe departure from Beirut of armed personnel, and to assist in the transition of authority to the Lebanese government in Beirut."

The second part of the announced role is reported to have roused concern in the Pentagon, which is said to be strongly opposed to any role beyond policing the departure of the Palestine Liberation Organization, and in Congress, where critics suspect the administration of attempting to skirt the 60-day limit for military deployment set by the War Powers Act.

The first part of the announced role involves providing escort service out of Lebanon for armed PLO guerrillas, who would be leaving a highly explosive situation under the guns of their bitter enemies in the Israeli and Christian military forces.

Scenarios for the departure under study within the U.S. government suggest the departure is likely to take 10 days to two weeks, even if all goes well. There is concern that some of the guerrillas, preferring martyrdom to flight, would hide out or decide to fight it out despite commitments to the contrary by PLO Chairman Yasser Arafat.

Almost as hazardous, and potentially more prolonged, would be the little-noticed second part of the announced assignment: assistance in the transition to Lebanese government authority.

Even after the departure of most or all of the PLO guerrillas, Beirut would remain a cockpit of conflicting factions with a history of antagonism: rival armies, militias and roving bands, to say nothing of the Israeli and Syrian armies still nearby until long-term arrangements can be made for their withdrawal from all of Lebanon.

The Lebanese Army of about 30,000 men is considered relatively ineffective, and will take time, training and equipment to come into its own.

The PLO, recognizing this, has made the involvement of an international force one of its negotiating demands. At one stage, it was thought that the 5,000 to 6,000 PLO fighters would be accompanied by their families and close relatives, estimated at 30,000 to 40,000 people. Present planning does not call for this, and the PLO is concerned about the family members' security as well as that of many thousands of other Palestinian civilians after the guerrillas' departure. Protecting them would be neither a quick or easy job.

The Lebanese government, for its part, would like help in policing the post-departure situation. And the United States, according to officials, is anxious that the Lebanese not fail this first test.

In the administration's deliberations, Secretary of Defense Caspar W. Weinberger and the career military headed by Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, are reported to have raised many questions from the first about the risks inherent in any U.S. military role.

Moreover, the Pentagon has been strongly opposed to any role going beyond that of policing or escorting the departure of the PLO, according to a source familiar with the discussions. "They don't want American troops to be left in that country for any period of time," the source said.

As of yesterday, according to a White House official, both parts of the originally announced charter are considered operative. However, some powerful members of Congress, including Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), have flatly objected to any role beyond escort service for the PLO.

Reagan administration officials have told members of Congress and reporters that the U.S. military role in the multinational force in Beirut is not expected to last longer than 30 days.

At the same time, however, State Department officials told the House Foreign Affairs Committee that its report to Congress under the War Powers Act is likely to be under a section of the law that would permit the unlimited deployment of U.S. troops, skirting a section of the law that limits the deployment to 60 days.

Rep. Clement J. Zablocki (D-Wis.), the committee chairman, protested in a letter to Reagan last Tuesday that such a maneuver "could only be interpreted as an attempt to avoid capriciously" the legal limitation on the troop deployment imposed by the War Powers Act.

"Such action would have incalculable effects on executive-legislative relations on a variety of foreign policy issues," Zablocki wrote.

The better-known provision of the War Powers Act--section 4 (a) (1)--requires a report to Congress when U.S. forces are introduced "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances."

A report under this section triggers the time-limitation provisions of the law. Under these, the use of the forces must be terminated within 60 days unless Congress votes a declaration of war or adopts some other measure extending the time.

The president, however, can take another 30 days if he certifies that "unavoidable military necessity" regarding the safety of the forces exists, and that he is bringing them home.

According to a 291-page study of the act by former committee aide John H. Sullivan, published by the House committee three months ago, a president can avoid the time limit if he can report a military deployment under a lesser known provision, section 4 (a) (2).

This provision is for reports whenever U.S. military forces are introduced "into the territory, airspace or waters of a foreign nation, while equipped for combat . . . . "

Zablocki argued in his letter that any common-sense assessment would conclude that "imminent involvement in hostilities" will face U.S. troops entering Beirut, and thus the stronger report is required.

State Department officials, on the other hand, argued that the prior agreement of the PLO, the Lebanese government, Israel, Syria and other parties to the withdrawal of the Palestinians would greatly reduce the danger. Therefore, the administration could appropriately dispatch its War Powers Act report under the nonbinding "equipped for combat" provision.