The first 350 French troops of a multinational peace-keeping force landed at Beirut's port today amid continuing confusion over plans for its deployment and the killing of an Israeli soldier in the central part of the city.

The troops are the first of 3,000 that France, the United States and Italy ordered to return here following the massacre a week ago of hundreds of Palestinian refugees.

The troops, which were evacuated before last week's refugee camp massacre, are again assigned to help the Lebanese Army maintain law and order in the capital.

U.S. officials said Britain and the Netherlands are interested in contributing troops to the force. Story, Page A16.

The vanguard's arrival came following a favorable reaction by most Moslem political groups here to the inauguration yesterday of Christian businessman Amin Gemayel, 40, as president. He pledged to maintain close ties with the Arab world and to work for the withdrawal of all foreign troops from the country.

In today's shooting incident, a gunman opened fire at point-blank range on an Israeli officer and three soldiers at a sidewalk cafe near the center of West Beirut. Police said the officer was killed and the others injured.

Six Israeli armored personnel carriers loaded with troops wearing steel helmets and flak jackets closed off the street and picked up a number of suspects. Shops slammed their shutters and soon the entire downtown area was deserted.

Last night, gunmen thought to be Moslem militiamen opened fire on an Israeli patrol near a former Palestinian guerrilla office on Corniche Mazraa, the main boulevard separating the southern and northern parts of West Beirut. Apparently, the Israelis sustained no casualties and the assailants got away despite an intense manhunt carried out under flares over the streets.

The incidents put a damper on the air of returning normality and put in question whether Israel would maintain its decision, announced today by Israeli radio, to withdraw from West Beirut by Sunday.

On hand to greet the French troops were French Ambassador Paul Marc Henry and a handful of Israeli soldiers. The issue of their removal was reported to have kept the French troops from disembarking yesterday as planned.

U.S. special envoy Philip C. Habib was reported to have pressed the question of removal of the Israeli forces with Prime Minister Menachem Begin and Defense Minister Ariel Sharon in Jerusalem today. The troops moved into West Beirut following the assassination of Amin Gemayel's brother, Bashir. Israel said then that the purpose was to impose order.

Henry briefly exchanged words with the colonel commanding the Israeli unit at the port and got a pledge from him to withdraw his troops, although not an Israeli medical team also located inside the port perimeter.

The ambassador was heard to say to the Israeli colonel, "You stay on your side and we'll stay on our side" of the port. Most of the French troops headed off to the French Embassy residence near the line dividing this city between its predominantly Moslem and Christian sectors.

It was still unclear where the roughly 1,000 French, 800 American and 1,150 Italian troops making up the force will locate or how long they will remain. Beirut radio reports said Gemayel was taking charge of negotiations among the three Western ambassadors here, the Israelis and the Lebanese government.

Henry indicated that he thought it would be months before the force left but said, "Our troops will stay until peace and security are established in the whole of Beirut. We will work with the Italian and United States contingents and stay as long as required."

His reference to "the whole of Beirut" appeared to be in reply to the Israeli assertion that its forces will remain in the southern districts, where the airport and Palestinian camps are situated.

President Reagan has said that Israel must evacuate all of Beirut, including the southern districts, but negotiations to achieve this objective apparently still have some way to go.

Diplomatic sources here said the United States, France and Italy were insisting that the bulk of the peace-keeping force must be concentrated in the southern district rather than central Beirut, where the Lebanese Army is already deployed, and that Israel should withdraw south of the airport.

President Gemayel began the search for a new prime minister after Shafiq Wazzan handed in his resignation. Under Lebanon's complex sectarian-based political system, the prime minister must be a Sunni Moslem and the president must be a Maronite Christian.

A broad spectrum of Moslem political leaders sent Gemayel congratulations and promised their support for his announced program of ending political violence.

Only the leftist Morabitoun, the main Moslem militia, withheld its blessings, saying in a statement it would not cooperate with any regime established under Israeli occupation.

Press reports here said the Lebanese Army prosecutor, Assad Jermanos, had begun an investigation into the massacre of Palestinian refugees with a visit to the Sabra and Shatila camps, and that Gemayel was fully supporting this despite the implication of the Phalangist militia in the killings. Gemayel's late brother was commander of the militia.

A spokesman for the International Red Cross said the discovery of a mass grave, containing 19 members of one Palestinian family, raised the total of confirmed deaths in the massacre to 317.

[The Associated Press quoted Lebanon's prosecutor, Camille Geagea, as saying that the total of bodies found by all search groups was 597].