Italian officials said today that a bomb appears to have been the cause of an explosion yesterday at the Bologna railway station that killed at least 84 persons and injured another 189.

A hole three feet long and eight inches deep was discovered in the pavement in a corner of the station's second-class waiting room, and local state prosecutor Ugo Sisti said that his office was treating the explosion as a possible criminal act.

Sisti said the hole, as well as unidentified "technical material" collected at the scene, led experts to conclude that a bomb was the most likely cause of the explosion. Investigators also found scraps of black plastic that they said could have been from a bag or suitcase containing an explosive device.

If the explosion was caused by a bomb, it would be the most serious terrorist act committed in postwar Italy. It comes at a time when many Italians believed terroist acts were subsiding because of an ongoing police crackdown.

Most terrorism here in recent years has been the work of leftist groups such as the Red Brigades, but some persons claiming to be Red Brigades members or from other leftist groups have called news organiaztions disclaiming responsibility for the Bologna explosion.

The rightist Armed Revolutionary Nuclei appears to be the major suspect at the moment, despite conflicting calls last night denying and claiming responsibility for the group's involvement. Just two hours before the explosion, a Bologna judge indicted eight rightists in an Aug. 4, 1974, bombing of a train in which 12 persons were killed. A caller to a Rome newspaper, La Repubblica, said the armed Revolutionary Nuclei was responsible and said the bombing was in honor on one of the defendants, Mario Tuti.

Today, a letter with the name of the rightist group was left for journalists near a statue in Naples denying responsibility.

"We do not strike at a heap. We have precise objects," the one-page message said. It threatened reprisals against journalists who write that the Armed Revolutionary Nuclei carried out the attack.

Prime Minister Francesco Cossiga visted Bologna's major hospital and its morgue today to console the wounded and the relatives of the dead. Later, Cassiga visited the site of the explosion where twisted girders, slabs of concrete and piles of cement rubble are all that remained of the station's left wing. Until yesterday, the wing housed first-and second-class waiting rooms, offices, a bar and a restaurant.

Most of the dead were Italians, officals said, but the bodies of a Japanese man, a Frenchwoman, a West German woman, her two children and a British man were found.

Two brothers from Provo, Utah, William S. Davis, 22, and Jeff Davis, 19, were listed among the wounded. William suffered a kidney injury and Jeff a leg wound. Their father, Garold N. Davis, said their conditions had improved today.

William "saw stuff flying by at 100 miles an hour -- chairs, bottles, debris" fell to his knees and covered his head, the elder Davis said. He said a stranger took off his shirt and wrapped a tourniquet around Jeff's injured leg.

Throughout the day, teams of firefighters, police and volunteers, many with face masks, continued to shift through the debris caused by the explosion, which some eyewitnesses said had literally lifted the building into the air before it crumbled into pieces.

Two empty train cars, blanketed in dirt and dust with their windows blown out, sat on the tracks. Passengers came and went as usual in the main lobby of the station, which was not affected by the blast.

Resue workers pulled a wounded child from the rubble. Officials said they did not know immediately whether the child, aged 7 or 8, was a girl or a boy but that the child had been trapped for at least 16 hours after the explosion.

Explosives experts said they believe that up to 88 pounds of TNT or other explosives must have been placed in the waiting room, but they say they are not sure whether a detonator was deliberatiely set off or whether a magnetic field created by the railway's electrical network accidently triggered the explosion. Investigators said the terrorist who may have placed the explosive could well be among the dead or wounded.

Inside the crater there were several wreaths, including one brought by Falminio Piccoli, secretary of the ruling Christian Democratic Party. A small bunch of carnations with a tag reading, "To my papa," was left by a small boy whose father was one of two taxi drivers killed when the front wall of the station collapsed.

The Bologna train station was jammed with travelers and vacationers at the time of the explosion, which brought down part of a platform roof into the main track and onto two cars of a Switzerland-bound train.

Doctors at the central Bolgna hospital and oficials at the station say that the corridors of both places have been jammed with weeping men and women worried about the welfare and the where abouts of relatives and friends. So far, only 55 of the bodies have been officially identified. One hospitalized man interviewed by Italian television told how he had been knocked out when the explosion blew him off the platform and when he regained consciousness, he could not find either his wife or his small son.

Bologna police set the offical death toll from the disaster at 76, but a spokesman for the state police in charge of the rescue effort at the station said 84 bodies were recovered. The station was crowded with passengers leaving the city at the start of Italy's annual vacatin season. Local police also said 189 persons were injured, with 17 in "very serious" condition.