Israel announced today that 90 people, including 75 Israelis and 15 Arabs, died in the explosion that destroyed the Israeli Army's regional headquarters in the southern Lebanese city of Tyre on Thursday.

The cause of the blast remained undetermined, amid growing indications that it may have been an accident rather than a guerrilla attack on the seven-story building.

The death toll is the highest that Israel ever has suffered in a peacetime disaster, and the government declared Monday a national day of mourning.

Rescue teams completed the task of clearing away the rubble and recovered the bodies of 28 more Israelis from the wreckage. The final death toll of 75 Israelis included soldiers and border police guards.

The victims also included Palestinians who were being held in the building for interrogation, and a number of Arabic-speaking Israelis who were stationed at the headquarters on the outskirts of the Lebanese coastal city.

The rescue teams had hoped to find some victims still alive, but only bodies were pulled from the rubble today. In addition to the 90 dead, 19 other victims remained hospitalized with injuries, including six who were said to be in critical condition.

The worst previous peacetime accident was when the Israeli submarine Dakar disappeared in the Mediterranean in 1968 with the loss of all 69 of its crew.

Senior Israeli officials at the scene in Tyre, including Defense Minister Ariel Sharon, have refused to speculate on the cause of the explosion, adding to the view that it may not have been an attack by the Palestine Liberation Organization, as many Israelis had at first assumed.

Tonight the Voice of Israel radio quoted a high-level source as saying that investigators in Tyre still had not discovered any evidence that the explosion was the result of a deliberate attack.

According to experts interviewed on Israeli television, the evidence that the collapse of the building may have resulted from an accident includes a lack of any sign of high-powered explosives in the structure and no signs in the area, such as broken windows in nearby buildings, that there had been a concussion explosion.

There was increasing speculation that the collapse of the apparently poorly constructed building may have followed an internal explosion set off by a gas leak.

PLO leader Yasser Arafat announced, however, in an interview broadcast Friday night by the Paris-based Radio Monte Carlo that the Tyre blast was the work of Palestinian guerrillas and their leftist allies still operating behind Israeli lines in Lebanon, the Associated Press reported.

The Israeli Army has appointed an investigative commission to determine the cause and report its findings this week.

Whatever the cause, the disaster has been a severe setback for the army and seems certain to increase domestic criticism of the continuing occupation of southern Lebanon. The toll increased the number of Israeli deaths from the war in Lebanon to about 445.

The decision to declare the day of mourning was said to have been reached in telephone consultations between Deputy Prime Minister Simha Ehrlich and Begin, who had flown to Los Angeles for the start of a speaking tour of the United States.

Begin subsequently cut short his U.S. visit, which would have included a meeting with President Reagan, to return to Israel immediately upon receiving word of the death of his wife in a Jerusalem hospital.