Eight hundred U.S. Marines, wearing battle fatigues and carrying black M16 rifles, will land in the port of Beirut next week to provide "a presence" as the Lebanese Army escorts about 7,000 Palestine Liberation Organization combatants out of the city, the Pentagon said yesterday.

The Marines, who last landed in Lebanon 24 years ago, will be authorized to shoot if fired upon, but otherwise to do little more than form part of the backdrop of foreign military forces for evacuation of the PLO.

The first 350 of 800 French troops arrived at dawn today at a Beirut port, and an Italian force is to arrive when the Marines do. The French were to start in the port and move to the heart of the city near the museum. The Italians are to land and remain at the Beirut International Airport, according to the Pentagon.

"Their presence lends a sense of security and stability," Pentagon spokesman Henry E. Catto Jr. said yesterday of the foreign troops. He added that the United States anticipates that the Marines' mission will be "totally pacific."

The Marines, while setting up positions near the port, would be in position to cover a hasty exit by foreign forces if the cease-fire agreement breaks down, military sources said.

If things do not work out peacefully, Catto said, the Marines can count on help from warplanes on the carrier Independence standing off Lebanon in the Mediterranean Sea.

Backup U.S. military forces would cover withdrawal of the Marines, but not by staging any offensive action since the international agreement hammered out by U.S. special envoy Philip C. Habib calls for foreign military forces to leave Lebanon if fighting breaks out again.

The Marines are scheduled to arrive in Beirut over three days beginning Thursday, according to a timetable released yesterday by the State Department.

The Marine amphibious assault ship Guam is the flagship for the operation, but neither the Guam nor four accompanying assault and cargo ships will take tanks, other armor or artillery into Lebanon.

The Marines will land lightly armed, with M16 rifles for the troops, .45-cal. pistols for the officers, some Dragon antitank missiles and the usual supplies and food.

Catto said the Marines will be equipped with tents and remain in the port area. He said they will not be digging for mines or engaging other forces in any way.

"This is not the kind of thing that can be rehearsed in advance," Catto said. "The key word has to be flexibility based upon what the Lebanese need and upon what the [U.S.] ambassador and Embassy staff feel is prudent and necessary."

Catto said there will be no overall military commander for foreign assistance forces. He said orders would flow from top authorities here to the U.S. military command in Europe and then to the naval task force off Lebanon.

The Marine force involved is the 32nd Marine Amphibious Unit attached to the Sixth Fleet in the eastern Meditteranean and commanded by Col. James Mead of Boston.

President Reagan said yesterday that the Marines will stay in Lebanon no more than 30 days, and the State Department timetable calls for their departure over a six-day period beginning Sept. 21.

"They are there as a presence," Catto said. "I foresee no interaction with the Palestinians. The Lebanese basically are the key factor in bringing this about."

Asked if the Marines could fire at Israelis if fired upon, Catto said, "The right of self-defense is not limited by nationality."

At another point, Catto said an individual Marine is authorized to decide on the spot whether to return fire and need not ask permission from higher authority.

Pressed on how Marines could help keep the peace in Beirut if they are to withdraw at the first sign that the evacuation agreement has been violated, Catto replied, "The withdrawing forces, realizing their rear is vulnerable, feel the need for assurance."

If the Marines do stick close to the port, as Catto indicated, they will witness the PLO forces boarding ships chartered by the International Red Cross. The Pentagon said that the cost of chartering the ships could reach $4 million and that the State Department is willing to help foot the bill.

Once aboard the ships, PLO forces will be required to turn over weapons to crews running the vessels. Many of them will sail to Cyprus before going to Jordan, Iraq, Tunisia, North Yemen, South Yemen, Syria, Sudan and Algeria.

Other PLO members will be flown out of Lebanon, and some may be moved to Syria on the Beirut-Damascus highway, which is to be patrolled by French and Lebanese troops.

The last time the Marines went to Lebanon was on July 15, 1958, on orders from President Eisenhower to save the Christian government from being toppled by Moslem insurgents.