A truck bomb exploded in crowded downtown Kabul Wednesday, killing six people and wounding 49 on the final day of celebrations marking the 1978 coup that brought the ruling party to power.

Brig. Gen. Saifullah, in charge of police in the Kabul area, said the truck had Pakistani license plates and he blamed the blast on U.S.-backed Moslem rebels fighting to topple the Soviet-installed government. He said terrorists timed the explosion to coincide with the 10th anniversary celebrations.Radio Moscow blamed the blast on "opponents of the people's government" who want to frustrate the Afghan peace pact signed recently in Geneva and opposed by the Afghan guerrillas.

No group claimed responsibility for the explosion, which echoed across the basin in which Kabul sits when it occurred about 12:30 p.m. near the Pamir movie theater as crowds thronged nearby markets. A gray pall of smoke covered the area.

The force of the blast destroyed the truck and hurled its tangled wreckage off the side of the road, down an escarpment and onto the bank of the Kabul River, which flows through the heart of the city.

Windows of nearby buildings were shattered, and several vehicles standing nearby were badly damaged. Shards of glass and pieces of metal littered the street, and the spot where the truck had stood was scorched black.

Scores of plainclothes and uniformed police and paramilitary troops cordoned off the area.

Saifullah told reporters six people four men, a woman and a girl were killed in the blast and 49 were injured, 27 of them hospitalized. He said a time-bomb was planted on the truck and he produced a Pakistani license plate he said was on the vehicle.