A bomb exploded on a bus in central Israel today and the Palestine Liberation Organization claimed responsibility, saying that the blast marked the start of increased military operations inside Israel.

Israeli authorities said eight persons were injured in the explosion, none seriously. The Palestinian news agency WAFA said several persons were killed the bomb.

The attack came as PLO leader Yasser Arafat and rightist Christian Phalange leader Pierre Gemayel, in separate statements, warned that increasing fighting among their forces in southern Lebanon could lead to an Israeli invasion and occupation.

Fighting and artillery exchanges continued throughout the day in the border area despite the scheduled implementation yesterday of the third stage of an agreement by which combatants were to withdraw and be replaced by regular troops of a rebuilt Lebanese army.

Arafat addressed an urgent appeal to Arab heads of state through the Arab League, warning of "The serious situation in southern Lebanon resulting from escalating Israeli military intervention."

Gemayel said Israel could use the continued Palestinian presence in southern Lebanon as a pretext to occupy the area. "If we do nothing about the situation, we would then in fact be inviting Israel to occupy the south," he said.

The bus damaged by the bomb was on its way from Kiryat Shmona, near the Lebanese border, to Tel Aviv. Them bomb exploded when the bus was in Afula, 60 miles northeast of Tel Aviv.

An Afula police spokesman said 11 Arabs who were aboard the packed bus or in the vicinity of the blast were detained.

The PLO said it is determined to escalate its guerrilla operations "everywhere in Palestine" in retailiation for Israel's decision earlier this week to extend Israeli regulations and social services to Arabs on the occupied West Bank and Gaza Strip.

"This decision fitted in with the fascist and aggressive nature" of the government headed by Israeli Prime Minister Menahem Begin, the Palestinian statement said.

Meanwhile, a spokesman for George Habash's hardline Popular Front for the Liberation of Palestine, said the front would try to organize a general strike in the West Bank and Gaza to protest the Israeli decision.