A booby-trapped car exploded today in the midst of a crowded security roadblock manned by Palestinian guerrillas on the coastal highway between the Lebanese capital and the southern port of Tyre.

Authorities at the scene of the explosion, which blew away the sand-bagged roadblock emplacements as well as destroying a nearby restaurant and some 15 roadside stands, said that 15 persons -- including seven guerrillas manning the roadblocks -- were killed. About 40 others, many of them children whose school bus had stopped at the roadblock, were injured.

The explosion on the outskirts of the village of Zrariyeh was the latest in a series of mysterious bombings in the past week and a half around this war-torn land. Previous explosions in other parts of the country have killed more than 40 people and wounded more than 140.

The sudden spate of bombings, a new twist in the violence that has racked Lebanon since its 1975-76 civil war, was seen by government authorities as a potential threat to the tenuous cease-fire worked out by Arab League negotiators last June to stifle violent battles between Lebanon's well-armed warring factions.

Today's bombing was reminiscent of one 11 days ago in the port city of Sidon, north of Zrariyeh in which a booby-trapped truck parked in front of the regional headquarters of the Palestine Liberation Organization (PLO) and its allied leftist forces, exploded killing 25 people and wounding 108 others. Most of the victims were civilians who had been in the crowded street at the time.

The Sidon bombing was followed in a matter of days by a series of similar incidents in a northern Lebanese cement factory, in the streets of a Shiite Moslem neighborhood of Beirut and, a week ago, in one of the capital's movie theaters, all of which caused another 18 deaths and 40 injuries. All these previous bombings were claimed by a mysterious group calling itself The Front for The Liberation of Lebanon From Foreigners in calls made to international news agencies here.

No one has yet taken credit for today's bombing outside of Zrariyeh, a Shiite Moslem village 10 miles north of Tyre. But PLO officials and Lebanese authorities noted that in its style -- a booby-trapped car -- and the choice of target -- Palestinian, Syrian, or leftist Moslem militiamen -- today's explosion was similar to the rash of bombings that began Sept. 17.

The shadowy Front for The Liberation of Lebanon From Foreigners has in the past few years claimed credit for dozens of attacks against Palestinian guerrillas in Lebanon or soldiers from Syria's 23,000-man peacekeeping force that has garrisoned large parts of the country since it imposed an end to the civil war five years ago. The group also claimed credit for a sniper attack on the bullet-proof limousine of former U.S. ambassador to Lebanon John Gunther Dean last year.

Because of the front's choice of targets in the past it has been suspected of being a secret Christian terrorist group. The PLO has claimed that the Front is nothing more than a cover for Israeli agents operating in Lebanon.

But Western intelligence sources and local security officials are not so sure. They point out that given the military forces aligned against the Christians in their enclaves stretching from east Beirut northward, they would be the last to want to precipitate a new round of fighting with their traditional Palestinian and Moslem enemies. In recent clashes the Moslems have been backed by the the Syrian peacekeeping force.

The thinking of many officials here is that the new bombings have been carefully orchestrated by those who would like to see Lebanon remain destabilized. This, Western diplomatic sources here say, could be anyone from the Israelis, who benefit from the continued instability of the country, to the Syrians themselves whose grip on Lebanon is justified only by the political chaos.

Another possibility which cannot be discounted, is that today's explosion is the result of continuing interfactional fighting between two rival groups within the 16-party leftist national movement which is allied with the PLO. The roadblock blown away by today's bombing had been set up to control the flow of arms and explosives in the area in an effort to stifle recent battles in the area between the Shiite Amal militia and the Lebanese Communist Party.