Lebanon's two-day cease-fire crumbled today as shelling resumed between Syrian troops and Lebanese Christian militia at Zahle, 30 miles east of here, and Christian artillery fired on Chtaura, on the main highway from Beirut to Damascus.

"Back to square one," a U.S. diplomat said, commenting on the renewed artillery duels around Zahle and random shelling in Beirut. The 48-hour cease-fire allowed evacuation of severely wounded residents of Christian-held Zahle after more than a week of heavy fighting, but the two sides were unable to reach an agreement to end the dispute over the city, located in the strategic Bekaa Valley.

Lebanon was rocked by futher violence overnight as major Israeli raids hit Palestinian sites from the Israeli-Lebanese border to Damour, about 12 miles south of Beirut, reportedly killing 15 persons and injuring 45.

Although the Israeli raids and the fighting in Zahle were not directly linked, the raids were interpreted here as signaling Israeli support for the right-wing Christian militias.

Buoyed by that support, Christian leaders appeared determined to press their case with world public opinion in an attempt to force the Syrians out of Lebanon, five years after they came here as an Arab Deterrent Force to halt Lebanon's civil war.

The Israeli raids, carried out by helicopter-borne commandos and aided by shelling from gunboats off the Mediteranean coast near Saadiyat, hit several sites that Israel said had been used to train Palestinians for attacks against Israel. The heaviest raids centered on Arab Salim, a Palestinian installation near the largely destroyed market town of Nabatiyeh.

This morning, six Israeli F4 Phantoms bombed a cluster of five Palestinian bases around Damour.

U.N. peace-keeping forces reported that Israeli gunners fired 650 rounds at Palestininian and leftist Lebanese positions at the Crusader-era Beaufort Castle while 440 rounds were fired back in Israel's direction.

Isreali military spokesmen claimed their forces destroyed two of the Palestinians' 60 Soviet-built T34 tanks -- an example of the type of weapons Israel has cited to justify its preemptive strike policy.

Military specialists here dismiss these World War II-vintage tanks as a threat to Israel's sophisticated armed forces and suggest they were acquired to balance the stepped-up Israeli supply of equally obsolete U.S.-built tanks to the right-wing Christian militias.

In Zahle, Lebanese gendarmes who entered the city of 150,000 reported militiamen manning the lines in great numbers. The gendarmes said the residents were staying in their cellars with large stacks of canned goods, but were running short of bread and water. The power has been off since fighting escalated on April 2.

The gendarmes said that both the Christian militia and the Syrian troops ringing the mountainside city were preventing the adult inhabitants from leaving.

In Beirut, Christian militia spokesmen said they were determined to fight on "although every one else wants to freeze the situation as usual."

Washington Post correspondent William Claiborne reported from Jerusalem:

Israeli Prime Minister Menachem Begin wanred today that raids such as those last night and this morning will continue in an effort to disrupt PLO training in Lebanon. He said: "Our soldiers once again demonstrated courage and dedication, going out to defend our people against those who plot to murder men, woman and children in our country, in the only fashion in which we can protect our people, namely by striking a preemptive blow."

Sources in the Israeli Army commanmd said the attacks were not connected to Syrian shelling of Lebanese Christian neighborhoods in eastern Beirut and Zahle but were launched on the basis of military intelligence indicating resumption of Palestinian training.

In the night attack near Nabatiyeh, the Army command said, one Israeli soldier was killed and one seriously wounded. An Army official said five Palestinian guerrillas were killed.

The taining base, the Army command said, consisted of two bunkers, a combination dormitory-ammunition depot, two tents and two trucks with mounted machine guns, all of which were destroyed.

Army intelligence said the base normally houses 40 to 50 guerrillas of Fatah, the main component of the PLO, but most of the guerrillas fled when the paratroop unit landed by helicopter.

Authorities refused to say how many troops participated. An Army source said, "Let's just say it was a big operation."

The official said the PLO's T34 tanks had not yet been used but are regarded by Israel as threatening to change the balance of power in southern Lebanon. T34s, he conceded, are not suitable for conventional tank warfare in the area's hilly terrain but could be used as "mobile cannon."

An Army official said the Nabatiyeh camp was attacked because the PLO had made it a permanent facility, with elaborate bunkers and buildings.