BEIRUT, JUNE 1 -- Lebanese Prime Minister Rashid Karami, a Sunni Moslem and 10-time premier of this war-shattered country, was assassinated today by a bomb that exploded at his seat on a military helicopter that was carrying him to Beirut from Tripoli, his home town in northern Lebanon.

There was no immediate indication of who was responsible, and figures from all factions of Lebanon's fragmented political leadership -- including Christians who had strongly opposed him -- condemned the slaying of the 65-year-old prime minister. Karami has been a close ally of Syria, and his death deprives that country of a major political player on the Lebanese scene.

All 14 occupants of the helicopter, including Interior Minister Abdullah Rassi, were injured in the explosion, which blew off the wheels and ripped off the doors. The wounded copilot took over from the more seriously injured pilot and made a crash-landing at a small airport north of Beirut.

Lebanese President Amin Gemayel called an urgent meeting of top Army officers and ordered military experts to investigate the bombing. Tonight, he named Selim Hoss, minister of labor and education and a former prime minister, to head a caretaker government until a new one is formed.

Earlier, Gemayel flew to the St. Martine Hospital in Christian-controlled Jubail, where Karami's body was taken. Upon hearing of Karami's death, Gemayel offered a moving eulogy for his prime minister, who had submitted his resignation on May 4 in the face of mounting Christian and Moslem criticism of his closeness to Syria and his efforts toward unity.

The president described Karami as "Lebanon's martyr along the course of suffering" and a man who "devoted his life to consolidate the country's political heritage." He ordered a state funeral for Wednesday.

In Damascus, Syrian Cabinet ministers observed a minute of silence during their meeting today and radio stations said flags will be flown at half-staff for three days. Damascus radio charged that "Israel and its agents in Lebanon" were behind the slaying.

{In Washington, the State Department condemned "in the strongest terms the senseless, criminal assassination," and called on the Lebanese people to "move forward in the spirit of moderation which Prime Minister Karami personified."}

The chief of Lebanon's Air Force, Brig. Fahim Haj, said that the helicopter, randomly selected from among five for security reasons, had been thoroughly searched before it left a military base north of the Christian seaport of Jounieh to pick up the Karami party at Bqaa Sefrine, near Tripoli, and that the pilot and copilot were informed of the identity of their passengers only 10 minutes before take-off. But luggage brought aboard by Karami and the other officials had not undergone security checks, Haj said.

Lebanese police said it appeared the bomb was planted under or behind Karami's seat. Some news accounts said it had been in a briefcase.

In Lebanon's recent turbulent history, the assassination of political leaders has triggered bloody sectarian backlashes, and there were fears here today that Karami's assassination could provoke further violence.

After the slaying in 1977 of Druze chieftain Kamal Jumblatt, the father of the present Druze leader, Walid Jumblatt, Druze mourners went on a rampage, killing an estimated 300 Christians at random.

In 1982, Christian militiamen entered the Palestinian camps of Sabra and Shatila and massacred hundreds of unarmed civilians to avenge the assassination of president-elect Bashir Gemayel, the brother of Amin Gemayel.

Karami had been the only effective Moslem politician upholding Syrian prestige in Lebanon and he was an influential figure in Syrian-controlled northern Lebanon, although he was one of the few Lebanese leaders without an armed militia.

Coroner Joseph Soto, who examined Karami's body, said the blast had caused widespread injuries. The cause of death was attributed to heavy internal bleeding.

Rassi was treated for minor wounds and face burns. The pilot, Maj. Antoine Boustany, was severely injured, state-run Lebanese television reported tonight.

Former president Camille Chamoun, a traditional foe of Karami's and a vitriolic censurer of his policies, condemned the assassination. "The country cannot be governed with such terrorist methods," he said.

The Lebanese Forces, the Christian militias, denounced the assassination "despite the divergence in views" they had with him. The Lebanese Forces had been pushing Gemayel to accept Karami's resignation and had criticized the president and prime minister for contemplating a reinstatement of the Cabinet. Gemayel had stalled in making Karami's decision to step down final in fear of not finding a replacement and further enfeebling his administration.

The Lebanese Forces described the killing of Karami as yet "another cycle in the chain of open-ended war against democracy in Lebanon." The Christian militias had made no secret of their resentment of Karami's sympathies to Syria, blaming him for Lebanon's economic woes and openly disapproving of his prolonged boycott of Gemayel's regime.

An estrangement between Karami and Gemayel began in January 1986, when Gemayel resisted the signing of a Syrian-brokered accord that called on the Christian militias to make major concessions to the Moslem community and extended to Syria the right of tutelage over political and military affairs here.

Karami led a delegation of Moslem ministers asking for Syrian intervention in west Beirut last February to put an end to fighting between militias in the Moslem-dominated half of the Lebanese capital. As a result, 7,000 Syrian troops moved into west Beirut in a bid to clamp down on militia anarchy and block any possible expansion of Palestinian guerrilla power.

Karami's father, Abdel Hamid Karami, was one of the founding fathers of the Lebanese republic and served briefly as prime minister in 1945. Rashid Karami had championed efforts to carve out a bigger share for Moslems in Lebanon's four-decade-old political system, which favors the Christian community, now a minority.

In Tripoli, where Karami's casket was received by throngs of mourners at the city's southern entrance, women wailed and children cried. Stunned Tripolitans climbed on rooftops to catch a last glimpse of their leader.

Karami was born in 1921, gained a law degree at Cairo University, and was named justice minister in 1951, four years before becoming prime minister for the first of 10 times. He is survived by two brothers and five sisters.