Israeli forces shelled Palestinian military positions and shantytowns in and near Beirut today in their heaviest bombardment in 10 days and blockaded the predominantly Moslem western sector of the city for the third day.

There was no visible movement in the negotiations over the future of the encircled Palestine Liberation Organization forces, and U.S. special envoy Philip C. Habib met Israeli Defense Minister Ariel Sharon against a backdrop of suggestions that the blockade was interfering with progress in the talks.

The Israelis prevented any trucks from entering West Beirut, including those bearing flour, fresh vegetables and gasoline. Private automobiles were permitted to pass through the port area, however.

Correspondents watched right-wing Christian militiamen, who are effectively the Israelis' allies, prevent a four-truck Red Cross convoy from entering the western sector. Lebanese government officials said an Israeli officer forced three engineers to remove a key piece of pumping machinery from the waterworks serving West Beirut and part of East Beirut.

An Israeli Army spokesman said that the blockade was aimed at "creating a certain tension" and not at starving out the inhabitants of West Beirut or blocking water supplies to them, Washington Post correspondent William Branigin reported.

PLO gunners shelled the Lebanese presidential palace at Baabda in the hills southeast of the capital, but eyewitnesses said that the daylong bombardment by Israeli heavy artillery and gunboats far overshadowed the Palestinian firing. The Israelis announced a cease-fire effective at 4 p.m.--the fifth cease-fire since Israeli forces invaded Lebanon on June 6--but firing continued for several hours afterward.

Israeli troops and armor on the southeast edge of the city advanced under the bombardment and reached the runways of Beirut's closed airport, eyewitnesses reported. Military specialists considered the advance of only psychological importance, however, because the Israelis have controlled the high ground dominating the airport for weeks.

The Israeli tactics seemed designed to increase pressure on the PLO to capitulate, both by demonstrating its difficult military position and by encouraging Lebanese Moslem civilians to urge it to yield and thus bring an end to the siege.

Random conversations in West Beirut, however, suggested that the bombardment and blockade primarily were spurring anti-Israeli sentiment. There seemed to be a groundswell of support for Moslem Prime Minister Shafiq Wazzan, who reiterated his refusal to negotiate as long as the blockade remained in force.

"The entire world, and especially the mediators, must realize that this blockade, aside from its human consequences, has paralyzed all government . . . functions and all diplomatic efforts to reach a peaceful solution for West Beirut," Wazzan told a news conference.

"I call upon the world to wake up to the realities of this criminal siege," he said. "The civilized world should know what it is to be without water or electricity."

Both the Israelis and the Palestinians accused the other side of starting today's shooting. The Israelis' bombardment sent plumes of smoke skyward in the frequently targeted areas stretching from the southern shantytown suburbs of Lailake and Burj Barajneh northward to Fakhani and the Arab University, where many PLO offices are located.

Wafa, the Palestinian news agency, claimed PLO forces had destroyed as many as 10 Israeli armored vehicles, a wrecking truck and a bulldozer.

The only confirmed major Palestinian feat of arms, however, was the shelling of the presidential residence. Foreign Minister Fuad Boutros' chauffeur was wounded and his car damaged while Lebanon's top diplomat was conferring with President Elias Sarkis.

Both men are Christians and have incurred Palestinian wrath by what the guerrillas consider their pro-Israeli policies.

The most important diplomatic development was the meeting between Habib and Sharon, who was reportedly accompanied by David Kimche, director general of the Israeli Foreign Ministry. Analysts assumed that the meeting was concerned with efforts to relax the siege or with talks concerning terms for a PLO evacuation of Beirut.

Israeli officials provided no details on the meeting, news agencies reported from Jerusalem.

Despite the Israeli Cabinet's demand, reiterated yesterday, that all PLO troops and officials leave Beirut, Israel is known to hesitate to launch an assault. It fears suffering the casualties and drawing the international criticism that heavy fighting in the city would be likely to ensure.

PLO Chairman Yasser Arafat signed a document Saturday agreeing to leave Lebanon with his guerrillas if certain conditions were met, but there were no reports of progress in working out an agreement on the conditions.

Residents of West Beirut, hardened by seemingly nonstop trials during the past seven years, scurried around today stocking up on food. Bottled water was also in great demand.

Many shoppers acknowledged that their homes already contained closets full of spaghetti, canned goods, batteries, gas lamps and other items in the standard Beirut survival kit.

What spurred West Beirutis' anger was the belief that the Palestinians, or at least their guerrillas, were well supplied with food and water and possessed enough electric generators to keep medicine and other perishables intact.

Gasoline was in short supply, with the various militias shooting in the air to ensure that their vehicles were served first. "If you want to fill your tank," complained one disgruntled civilian, "bring your Kalashnikov," the omnipresent Soviet-made assault rifle.

Although the residents took the new measures seriously, especially the cutting off of water, electricity since the invasion has been so haphazard that residents have become used to doing without it. Many buildings and offices bought generators long ago, but gasoline to power them is now in short supply.

A United Nations Security Council resolution that condemned the blockade and urged restoration of shipments of essential supplies buoyed spirits somewhat in the besieged area. The resolution, adopted unanimously last night, did not mention Israel.

Arafat today took exception to a cabled suggestion from Libyan leader Muammar Qaddafi that the Palestinian forces "commit suicide rather than accept disgrace."

Arafat replied: "I am surprised by the tone of desperation that your message took." He also noted that the Arab world had remained silent about the PLO's plight, except for demonstrations in the Israeli-occupied West Bank.

Branigin also reported from East Beirut:

It was apparent from a vantage point overlooking the scene of the fighting today that there was little if anything to stop the Israelis from seizing the airport outright if they chose or driving up through the nearly deserted southern suburbs and Palestinian camps to the edge of West Beirut.

Using high-powered binoculars, telescopes and other advanced spotting equipment installed in a corner of a playground at a Maronite Christian school near the hilltop Baabda presidential palace, the Israelis directed heavy artillery fire at what they said were Palestinian guerrilla positions below. The Palestinians were firing back with bazookas, machine guns and some artillery of their own.

The Israeli firing went on for hours after a 4 p.m. cease-fire declared by Israel, despite a seemingly light reply from the other side.

"There is a break in the cease-fire, but we are still waiting for the outcome of the negotiations," Israeli Army spokesman Col. Paul Kedar told reporters today.

At a briefing at his Baabda headquarters as the sounds of Israeli and Palestinian shelling reverberated in the background, Kedar also denied that the Israeli blockade of West Beirut was intended to deny the mainly Moslem Palestinian controlled half of the city food, water, medical supplies and other provisions.

"There is no intention of starving out the population or closing water supplies," Kedar said. He said the blockade, imposed Saturday, was aimed at "creating a certain tension" and ensuring that the current impasse in the negotiations "does not develop into a new status quo."

However, Israeli military officials could not explain why trucks bearing produce have been prevented from entering West Beirut along with most other traffic and why water and electricity appear to have been cut off.

At the Israeli roadblock at the capital's main east-west crossing point--reinforced today by two tanks--an Israeli lieutenant contradicted the official spokesmen.

"Personally I would prefer that they the Palestinians surrender for lack of food," he said. "It's better than fighting, and they won't starve to death." As for the estimated 500,000 Lebanese inhabitants remaining in the western half of the city, "they are free to leave," the lieutenant said.