Separate attacks on two buses more than 100 miles apart during the weekend have underscored to Israelis the dangers that still exist despite the defeat of the Palestine Liberation Organization forces in southern Lebanon.

No Israelis were killed in the incidents, but 21 Israeli soldiers and 12 civilians were hurt. The first attack was Friday night against an Army bus south of Beirut. Last night, two grenades were thrown at a civilian bus in Tel Aviv.

The first attack was part of growing violence facing the Israeli Army in Lebanon and the second a disquieting reminder that even civilians in Israel's largest city are not immune to random attacks.

The Cabinet received reports on both incidents today. Afterward, a senior official blamed the increase in the number of attacks on Israeli soldiers on efforts by the PLO to undermine the troop withdrawal negotiations between Israel and Lebanon.

Meanwhile, police in Tel Aviv searched for those responsible for the grenade attack on the civilian bus. One of the grenades was thrown through an open window and exploded inside the bus.

Police arrested dozens of Arabs after the attack and reportedly released many of them after questioning. The state-run Voice of Israel radio reported tonight that police believe more than two men were involved in the assault.

The attack, the first on a civilian target in Tel Aviv since 1975 according to the Jerusalem Post, renewed fears of a possible surge of terrorist incidents. Most of Israel has been free of such incidents in recent years.

In a telephone interview today, Rafie Eitan, Prime Minister Menachem Begin's adviser on terrorism, said he believes the Tel Aviv bus attack was an "isolated incident." All evidence points to it being a "terror attack" by Palestinians, he said, but the number of Israelis killed in such attacks in recent years has steadily declined and he expects this trend to continue.

While Israel's pervasive security network has reduced the number of incidents in the country, the Israeli Army appears to be facing a far more serious situation in Lebanon. The attack on the Army bus Friday night by guerrillas armed with rifles, submachine guns and bazookas raised the Army's toll in Lebanon during the past three weeks to six dead and 24 wounded.

According to a military spokesman, since Sept. 1, when the PLO pullout from Beirut was complete, 104 Israeli soldiers have been killed and 203 wounded in Lebanon. This includes the 76 killed and 27 wounded in an explosion at the Israeli Army regional headquarters in Tyre, Lebanon, which an Israeli investigation said was caused by a gas leak.

It does not include the civilian truck driver who was kidnaped and killed after delivering supplies to an Army unit, or the injuring today of a border police guard in a sabotage incident near Tyre.

The 104 killed since Sept. 1 represent more than 20 percent of all Israeli military deaths as a result of the war in Lebanon.