Israel stepped up its pressure on the besieged western sector of the city today with air, land and sea bombardments, and the main leftist Lebanese ally of the Palestinian guerrillas told their leaders "to face reality" and leave in order to save the capital from destruction.

But Palestine Liberation Organization leader Yasser Arafat was reported today to be refusing to comply with the basic Israeli, and increasingly now Lebanese, demand that he and his top aides go abroad and order the 5,000 guerrillas trapped here to lay down their arms.

Israeli jets also bombed the densely populated, low-income district of Corniche Mazraa, where the Palestinians and their leftist allies have some offices. First reports said at least 50 buildings were damaged, some extensively, and it was feared that hundreds were killed.

"We don't know yet how many casualties there are, but there must be dozens of people buried in the rubble," said a man directing rescue operations quoted by The Associated Press. Eyewitnesses said the whole street looked as if it had been flattened by an an earthquake.

Arafat and his top aides held a long meeting last night with Lebanese Prime Minister Shafiq Wazzan to discuss three questions put to the Palestinian leadership earlier in the day by U.S. Middle East envoy Philip C. Habib. But Lebanese and Palestinian sources indicated today that the Palestinian leader had shown no softening in his demand for Israeli withdrawal from Beirut nor any willingness to lay down arms.

"They still think a miracle could happen in this bloody Arab world" to save them, said Walid Jumblatt, leader of the loose coalition of 13 leftist factions fighting alongside the Palestinians. "But I don't believe it.

"They should quit and not destroy Beirut," he said in an interview at his home with a small group of Western reporters as Israeli jets screeched over the city dropping bombs and Palestinian fighters answered with antiaircraft fire.

"Whatever happens," he said, "they must change the leadership of the PLO."

This clearest sign yet of a split between the Palestinians and their remaining Lebanese supporters came as Israeli forces fought their way to the strategic Beirut-to-Damascus highway in the mountains east of the capital and their warplanes clashed for the first time in 13 days with Syrian jets. Israel later said it shot down two Syrian Migs.

Throughout the day, Israeli jets, tanks, artillery, warships and troops battled against the combined forces of the Syrians, Palestinians and their Lebanese allies east of Beirut in what appeared to be the heaviest fighting in at least a week.

A senior Israeli official said after a Cabinet meeting in Jerusalem Thursday that Israel remained resolved not to send its invasion forces into West Beirut, news services reported from Jerusalem.

The official, who asked not to be identified, said, however, that Israel was refusing guerrilla demands that it withdraw its forces six miles from their present positions encircling Beirut.

Israeli also repeated earlier demands that the Palestinian guerrillas must lay down their arms and leave Beirut.

This morning, a war communique from Damascus said the Israelis still had not broken through Syrian positions guarding the Damascus highway. But by nightfall, various Lebanese radio reports and other sources said their tanks had entered Sofar and Bhamdoun, roughly 12 miles east of the capital.

This would mean that Syrian forces farther west on the highway at Alayh were trapped and that Israeli control over the 20-mile stretch between Beirut and the Bekaa Valley was now almost complete.

While heavy fighting along the Damascus highway continued into its third day, the Israelis escalated their pressure on the Palestinians inside the 10-square-mile sector of predominantly Moslem West Beirut without actually entering it. Israeli jets hit Palestinian quarters on the southern outskirts and even at the edge of the city's hilly central district. At the same time, Israeli warships pounded away at Palestinian and Syrian defenses along the seaside boulevard below.

Among the buildings hit was the New Mallas in Manara, where many Western diplomats, U.N. officials and correspondents live. One shell scored a direct hit on Time magazine correspondent William Stewart's apartment, causing considerable damage to the interior but no injuries. Another shell did some damage to the building where Los Angeles Times correspondent Michael Kennedy lives, but there were also no immediate reports of injuries there.

The Israeli warships apparently were trying to knock out the seaside positions of a pro-Syrian militia group known as the "Arab Knights" near the New Mallas building.

The target seemed one more indication that the Israelis still are determined to drive the Syrians as well as the Palestinians out of the city to end their influence in Lebanese politics. Their tactics also seemed aimed at achieving what Jumblatt called "a psychological atmosphere of surrender" even if Israeli forces did not directly assault the western sector.

Conditions in West Beirut continued to deteriorate today with the announcement that the water supply is being reduced to only four hours every two days. Already, there is little electricity, and all telephone lines to predominantly Christian East Beirut and most of those to the outside world have been cut.

The sense of impending doom was made even more acute today as the U.S. Embassy closed down all operations at the chancery along the seafront.

Most embassy employes were transferred to the residency at Yazieh, on the mountain slopes east of the capital.

Meanwhile, American nationals were evacuated on a boat from Juniyah, 10 miles north of Beirut, after being told Tuesday to leave the beleaguered eastern sector immediately. The British also sent a ship today, although its embassy near the U.S. mission remains open.

Meanwhile, cars packed with explosives and left in crowded streets have continued to cause enormous damage and casualties, adding to the already high tensions prevailing here. Yesterday, an explosion near a munitions depot of the Syrian-controlled Palestine Liberation Army near the harbor, killing at least 30 persons and wounding at least 100.

The effects of the mounting pressure on Lebanese leaders seemed clear in the interview today with Jumblatt, who said he had lost all faith in the National Salvation Council and would probably withdraw from it shortly.

He said the council, which held no meeting today, was scheduled to hold a session with Arafat shortly during which he expected it to press the Palestinian leader to leave the capital and go abroad, thus accepting the end of the PLO as a fighting force in Lebanon.

He said Bashir Gemayel, leader of the Lebanese Forces and one of the council's seven members, had held a direct telephone conversation with Arafat yesterday to discuss the Israeli and Lebanese demand for his departure.