Lebanon's prime minister narrowly escaped assassination this morning when a car filled with mines exploded about 50 yards from his motorcade.

Shafiq Wazzan was entering the outer gate of Government House in busy downtown West Beirut when the blast went off, blowing the roof, doors and floor out of the explosives-laden Honda station wagon his convoy had just passed.

Police said later that only about 10 pounds of the roughly 150 pounds of antipersonnel and antitank mines in the Honda had been detonated by a TNT charge.

Had the entire load gone off, a police spokesman indicated, it likely would have caused serious damage and casualties. As it turned out, no one was injured in the blast.

The attack pointed up again the vulnerability of the fledgling Lebanese government to such guerrilla actions. It occurred in an area where there is an unusually heavy concentration of Lebanese soldiers who guard the prime minister's offices and the nearby Central Bank and Information Ministry.

Lebanon's military prosecutor said descriptions of the man suspected of bringing the mine-laden Honda to the area were broadcast to border posts, seaports, airports and army and police stations. Prosecutor Assad Germanos said four persons were arrested on suspicion of involvement in rigging the car.

Unlike many previous bombings here, there was no group calling foreign news agencies to claim responsibility. The reason for the attack was something of a mystery.

Wazzan is a Sunni Moslem and a strong defender of the Israeli-Lebanese accords. He has been a staunch promoter of efforts to heal the rift between Moslems and Christians here.