President Reagan has assured Congress that the 800 Marines sent to Lebanon will not end up in combat there, and yesterday told the landing party securing the port of Beirut that its role "is crucial to achieving the peace that is so desperately needed in this long-tortured city."

"I want to emphasize that there is no intention or expectation that U.S. armed forces will become involved in hostilities," Reagan said in his letter to Congress explaining the mission of the Marines, who landed in the port of Beirut at 5 a.m. (Beirut time) yesterday.

The Lebanese greeted the Marines with applause, according to the Pentagon.

Reagan emphasized the peaceful side of the Marines' role in a letter sent to Congress Tuesday. The notification is required by the War Powers Resolution, which was passed in the Vietnam era as Congress sought a tighter grip over the deployment of U.S. military forces by the president in situations short of all-out war.

After telling Congress that the Lebanese want the 800 Marines, 800 French troops and 400 Italian soldiers to provide an international presence while Palestine Liberation Organization combatants leave Beirut, Reagan wrote:

" . . . Our agreement with the government of Lebanon expressly rules out any combat responsibilities for the U.S. forces.

"All armed elements in the area have given assurances that they will take no action to interfere with the implementation of the departure plan or the activities of the multinational force . . . . The departure plan makes it clear that in the event of a breakdown in its implementation, the multinational force will be withdrawn.

"Although we cannot rule out isolated acts of violence, all appropriate precautions have thus been taken to assure the safety of U.S. military personnel during their brief assignment to Lebanon."

Administration officials have said the Marines will stay no longer than 30 days.

"This step will not by itself resolve the situation in Lebanon," Reagan added, "let alone the problems which have plagued the region for more than 30 years. But I believe that it will improve the prospects for realizing our objectives in Lebanon:

"A permanent cessation of hostilities.

"Establishment of a strong, representative central government.

"Withdrawal of all foreign forces.

"Restoration of control by the Lebanese government throughout the country.

"And establishment of conditions under which Lebanon no longer can be used as a launching point for attacks against Israel.

"I also believe," Reagan concluded, "that progress on the Lebanon problem will contribute to an atmosphere in the region necessary for progress towards the establishment of a comprehensive peace in the region under Camp David, based firmly on U.N. Security Council Resolutions 242 and 338."

The president did not specify which section of the War Powers Resolution empowered him to send in the Marines. White House and State Department spokesmen said the resolution "does not require citation of any particular subsection."

Reagan told Congress he was acting "pursuant to the president's constitutional authority with respect to the conduct of foreign relations and as commander-in-chief of the . . . armed forces."

Pentagon spokesman Henry E. Catto Jr. said yesterday that the Marines went ashore between 5 and 6:30 a.m. Beirut time without incident. As the Marines relieved the French force, the tricolor French flag was lowered from grain elevators towering over the port and the Lebanese flag raised in its place by the Marines.

Noting that all was quiet at the port, Catto said the Marines' presence is "largely symbolic, and we hope it will remain so."

The 32nd Marine Amphibious Unit manning the port is armed and authorized to fire in self-defense, but not to engage in "sustained combat." Besides screening the departure of PLO forces from the port, the Marines would establish an escape corridor for the international forces if they had to depart suddenly.

While the Marines keep the port under control, the French force of 800 men is patrolling around the museum in the heart of Beirut. The 400 Italians are expected to land at the Beirut International Airport today and begin patrols there.

Before returning from Los Angeles to his mountaintop ranch north of Santa Barbara for two more weeks of vacation, Reagan sent this message of appreciation to the Marines:

"You are about to embark on a mission of great importance to our nation and the Free World. The conditions under which you carry out your vital assignment are, I know, demanding and potentially dangerous. You are asked to be once again what Marines have been for more than 200 years -- peacemakers.

"Your role in the multinational force -- along with that of your French and Italian counterparts -- is crucial to achieving the peace that is so desperately needed in this tortured city.

"I expect that you will perform with the traditional espirit and discipline for which the Marine Corps is renowned. Godspeed."

One of the platoon leaders with the 32nd Marine Amphibious Unit is Lt. Robert H. Barrow VI, 24, son of the Marine Corps commandant.

In Los Angeles, White House deputy press secretary Larry Speakes was asked if he had managed to get some Marines huddled around a radio to listen to the president's words. "If we had thought of that," Speakes said with a smile, "we might have done it."