Five persons, including two Lebanese policemen, were killed here tonight when shells slammed into downtown areas of Christian East Beirut for the second time in less than a week. Stray shells struck U.S. Marine positions.

Spokesmen for the Lebanese Forces Christian militia said Moslem Druze militiamen whom they are fighting in the hills overlooking Beirut fired more than a dozen rounds into the eastern sector of the capital during a 40-minute period before both sides agreed to a cease-fire.

They said the policemen were killed when a shell exploded close to the headquarters of the Surete Generale, in the Badaro neighborhood. Three civilians in a car traveling near the headquarters were also killed. Reports here said 44 other persons were injured.

Beirut Radio reported that shells hit Hadath, near a U.S. Marine position at the Lebanon University science building, and Baabda, where the presidential palace is located. Other shells reportedly fell near Marine positions around Beirut Airport.

"We did have a couple of stray rounds drop into our positions," Marine Capt. Dale Dye said, emphasizing that the shells did not appear to be aimed at the Marines.

Christian militia and Druze leaders accused each other of starting the shelling.

The fighting in the hills came amid announcements here today that France planned to airlift reinforcements to the multinational force and that a 100-man British contingent, scheduled to be in place on Tuesday, would establish positions on the edge of villages embroiled in the sectarian fighting.

The British commander, Lt. Col. John Cochrane, told a news conference his men will patrol a road that now serves as the Israeli Army's main supply route and is close to the scene of several confrontations between U.S. Marines and Israeli troops.

In Washington, State Department sources provided details of the U.S.-Israeli agreement reached Thursday on a line of demarcation meant to prevent such confrontations.

The sources said the line starts near Khaldah and proceeds north along a railroad track that runs into Beirut. The demarcation line forms a semicircle around the U.S. Marine position at the Science Faculty at Lebanon University and meets the Italian peace-keeping forces' area of operations to the north.

A British advance party has established a headquarters in a tobacco factory complex about a mile southeast of the U.S. Marine checkpoint where an American captain drew his pistol to stop three Israeli tanks from entering a Marine position.

Cochrane said there would be no line of demarcation between his men, experienced troops who have just completed two years' service in Northern Ireland, and the Israelis.

But unlike the commander of the Marine contingent, Cochrane said he would be able to conduct liaison with the officers in the occupying Israeli Army and has already had one impromptu meeting with them.

French President Francois Mitterrand's decision today to reinforce the 1,600-man French contingent by about 350 more Marines followed two attacks that wounded three French Marines during the past week. It was interpreted here as a strong symbolic statement that his country would not be intimidated or deterred from staying here by the attacks.