Israel and Palestinian guerrillas exchanged artillery and rocket fire today in some of the heaviest shelling in the current cross-border flareup and Israeli planes attacked Palestinian positions in southern Lebanon.

The fighting presisted as U.S special envoy Philip C. Habib traveled to Beirut and then on to Saudi Arabia in his quest for truce, underlining the scant effect his appeals for a cease-fire have had so far and the limits of U.S. influence on the Israeli government and the Palestine Liberation Organization.

Five times during the day, while Habib met with Lebanese leaders, Palestinian guerrillas fired artillery and mortars into the southern Lebanese secessionist enclave controlled by Israeli-supported Christian militias and unleashed salvos of Katyusha rockets into the Galilee region of northern Israel.

Israeli gun emplacements along the border and inside Lebanon answered with heavy bombardments of the southern Lebanese towns of Nabatiyeh, Arnoun and Hasbaya -- all Palestinian-controlled -- and the Palestinian stronghold at Beaufort Castle just north of the Christian enclave, according to the U.N. Interim Force in Lebanon (UNIFIL).

The U.N. command at Naqura, in southern Lebanon, said that in one exchange the Israelis fired 400 rounds at Palestinian positions, while the guerrillas fired 250 morat and artillery rounds at the Marjayoun area, in the secessionist enclave, and 60 rockets into northern Israel. No casualties were reported.

Israeli fighter-bombers this afternoon attacked a Palestinian position where the Zahrani River runs into the Mediterranean and bombed a recently cut trail in the vicinity of Qasmieh, the Israeli Army command said. The trail, an Army spokesman said, had been intended for use by the guerrillas to bypass a bridge over the Litani River that had earlier been destroyed by the Israelis.

[PLO headquarters in Beirut said the rocket barrage hit 15 villages and settlements in the northern Israel, Washington Post correspondent David B. Ottaway reported. A CBS television crew said the Israeli bombing also aimed at a makeshift bridge thrown up across the Litani to replace that bombed earlier. The Israeli bombs hit three trucks and four cars waiting at the approches, killing about 20 passengers, all civilians, Ottaway quoted the televison crew as saying.]

In the early morning hours, Israeli gunboats shelled Palestinian targets along the caost between the port cities of Tyre and Sidon, the Army command said. But the Army spokesman denied PLO reports in Beirut that an Israeli armored column, supported by helicopter-borne troops, had attempted an attack near Beaufort Castle and been replused by guerrillas.

The spokesman also denied a report from the PLO in Beirut that an Israeli soldier was captured during an attempt Monday to alnd a force on the coast south of Sidon.

Prime Minister Menachem Begin, in an apparent attempt to disassociate himself from the notion that Habib's peace-seeking shuttle might imply indirect Israeli contact with the PLO, sid that the term "cease-fire" did not figure at all in his discussions yesterday with the U.S. envoy.

Begin, while touring shelled towns and settlements in northern Israel, reiterated Israel's position that Habib was authorized by the Israeli Cabinet only to meet with Lebanese President Elias Sarkis to discuss establishing peaceful relations between Israel and Lebanon. Peaceful relations with Lebanon, in the Israeli view, would mean an end to attacks against Israel from within Lebanon, in exchange for which Israel would call off its attacks.

Although Habib's terse statement after a meeting with Begin last night specifically mentioned a cease-fire, the Israeli Cabonet's communique did not use the terma nd , in fact, emphasized that Israel had not authorized anybody to conduct negotiations directly or indirectly with the PLO.

[Although PLO chief Yasser Arafat has expressed willingness to accept a cease-fire under certain conditions, a more militant guerrilla group, the Popular Front for the Liberation of Palestine, said in a statement today that it "will not respond to the cease-fire call," Ottaway reported. The Syrian government, he added, has not indicated whether it is read for a truce, but in the meantime is bringing radar guided artillery into Beirut to defend the Lebanese capital against Israeli air attacks.]

Israeli sources said Begin has become acutely sensitive to the appearance of a political victory for Arafat as a result of the Israeli Air Force's bombing of a residential area of Beirut, which claimed 300 lives by Lebanese government count. By drawing Israel into a position where it appears to be negotiating through a third party with the PLO, Arafat may have achieved a degree of de facto recognition that he did not have before, the sources said.

Begin, according to close aides, was eager to extricate himself from this dilemma by making the definitions in the Cabinet communique as vague as possible. He was said to be displeased that world-wide interpretation of the Cabinet decision focused upon a possible Israeli-PLO cease-fire mediated by Habib.

Similarly, the government took pains today to deny a report that the International Red Cross earlier this week arranged a brief truce between Palestinian guerrillas and the Israelis near Sidon so 20 bodies of civilains could be removed from underneath a Zahrani River bridge that collapsed in an Israeli bombing raid.

There has been an incipient debate in Israel over whether the government has been baited by the Palestinains into an intractable situation from which it cannot extricate itself without enhancing the PLO's political position.

Former prime minsiter Yitzhak Rabin today suggested that Israel will have to pay a steep "political price" as a result of its severe attacks in Lebanon. Begin's strategy, Rabin said, can never silence the Palestinian guerrillas' guns, but it has caused severe damage to U.S.-Israeli relations by having "reminded the Americans of Vietnam." Moreover, Rabin said, the bombing has caused Israel to lose its "moral advantage" inits long struggle against Palestinian terrorism.

A similar argument was advanced by about 20 leading Israeli intellectuals, most of them university profrssors, who wrote to the prime minister today asking an end to the bombing. However, another petition signed by 250 Israelis, many of them prominent businessmen, urged the government to protect the lives of Israeli civilians by any means. The letter said that if the PLO places its headquarters in densely populated residential areas it should not be Israel's fault that civilians are killed.