The cease-fire in Lebanon collapsed today as heavy fighting raged in and around Beirut, and Israeli troops moved tonight into Baabda, the site of Lebanon's presidential palace and a key point overlooking the main highway out of Beirut.

The Israeli advance into Baabda, without resistance from Lebanese Army units, was reported on state-run Lebanese radio and television and confirmed on the scene early Monday by Washington Post correspondent David B. Ottaway.

Earlier, employes of a hospital and of a police station in Baabda were reached by telephone and said that Israeli forces had entered the town. The development means that the Israelis are on the verge of taking effective control of the Beirut-to-Damascus highway and bottling up the Syrian, Palestinian and Lebanese units still in the western sector of the Lebanese capital.

The reported Israeli advance came after a day of fierce and apparently successful Israeli assaults on the strategic Khalde road junction south of Beirut's international airport. Israeli jets pounded Palestinian positions throughout Beirut again today.

Each side accused the other of breaking the cease-fire, which had brought a pause last night in the fighting that followed Israel's invasion of Lebanon last Sunday.

Late accounts suggested that the presidential palace--located in rolling hills five miles east of Beirut--had not been captured and that President Elias Sarkis was still inside.

The town of Baabda is controlled on the south by the Lebanese Army and on the north by the Christian-Maronite militia commanded by Bashir Gemayel, who in the past has cooperated with the Israelis.

Since the invasion began Sunday, Gemayal has been careful to avoid any suggestion of connivance with the Israelis, despite their reiterated efforts to picture him as the potential winner in a restructured Lebanon that Israel says it wants to bring about.

Lebanese sources confirmed today that Syrian units had been hit in Israeli attacks along the vital highway to Damascus yesterday and today despite a cease-fire that Damascus and Jerusalem agreed to Friday. That cease-fire halted the heavy artillery exchanges and air battles that had flared early last week and was followed by statements from Israel and the Palestine Liberation Orgranization last night that their forces were being ordered to halt fighting.

While Israel charged radical Palestinian guerrillas were deliberately undermining the truce, Salah Khalaf, who is also known as Abu Iyad and who is a leading figure in the PLO, said the Israelis "were just making excuses to terminate the cease-fire and complete their plan, which is to encircle the Palestine Liberation Organization leadership and trap it in Beirut."

Israeli forces were reported to have used the renewed fighting to overcome stiff resistance by Palestinians and by Lebanese Moslem militia units at the Khalde road junction and then to move armor columns through twisting back roads in the mountains above the junction toward the Beirut-to-Damascus highway.

"The Israelis are trying to establish the Damascus road as the new red line," said one Lebanese intelligence source.

He was referring to the line Israel drew across the map in southern Lebanon in 1976, limiting both the Syrian troops' proximity to the Israeli-Lebanese border and denying them effective air defense.

The cease-fire, which took effect at 9 p.m. 3 p.m. EDT yesterday had seemed to hold through most of the night within Beirut's city limits. Lebanese leftists in the front lines near the Khalde crossroads said that fighting there had continued sporadically throughout the hours of darkness, however.

When this correspondent neared the crossroads at 9:30 a.m., his car was passed in the other direction by six Syrian tanks moving in reverse gear at high speed, their guns pointed due south. Within one minute, two Israeli artillery rounds exploded in the sand 100 yards from the car alongside the airport runway perimeter.

Shortly afterward, artillery duels flared between the Israelis and the Palestinians and their allies just down the mountainside from Souq Gharb, a town on the Israeli line of advance.

Selim Kobeissi, an employe of a British bank in Beirut, said the cease-fire was broken there at around 10 a.m.

But he insisted that its intensity "was nothing compared" to the pounding the Palestinians and their allies had taken Friday and Saturday.

By this morning, the Israelis had advanced to Kaifoun, just four miles south of the Druze City of Aley, which abuts the Beirut-to-Damascus road.

Syrian armor and troops, especially the elite special forces, were fighting alongside the Palestinians in the mountains near Beirut.

Lebanese Army sources said that the Syrians had begun pulling out their last front-line units from the Beirut area yesterday when they came under attack by Israeli warplanes and were forced to return to battle positions. Their line of retreat effectively had been cut off.

Throughout the day, Israeli artillery and naval gunfire continued to rake the Khalde crossroads, around the airport, the once teeming but largely deserted Palestinian refugee camp, and the Shiite Moslem shanty towns nearby.

By midafternoon, Israeli warplanes had resumed their bombing runs and were attacking regularly on the half-hour until dusk.

Lebanese Army sources asserted that "at least 1,500" residents in Beirut, most of them civilians, have been killed or wounded in the fighting. There was no independent confirmation of casualty totals.

Military sources in Jerusalem claimed, but did not announce, that "hundreds" of PLO guerrillas surrendered south of Beirut after fierce artillery barrages in the Khalde area, correspondent William Claiborne reported.

Lebanese Army sources said that Israeli warplanes yesterday had dropped cluster-bomb units, an anti-personnel weapon that explodes before reaching the ground and covers a wide area, on the Borj Brajneh Palestinian refugee camp near the airport.

Correspondents who visited a Palestinian camp at the southern end of the city today reported bodies being buried 30 deep in mass graves.

Among the victims of yesterday's violence was Henk Velders, vice president of the local Chase Manhattan Bank branch, who was killed by a shell or rocket in his apartment near the centrally located American University of Beirut overlooking the Mediterranean. Velders was a Dutch citizen.

Hospitals in predominantly Moslem West Beirut were reported strained for beds and medical supplies.

Hospitals in Christian East Beirut were reported ready to handle casualties but only if they were neither Palestinians nor combatants.

Meanwhile, the French and Italian embassies announced separately that they were sending the liner Azur and the naval vessel Caorle, respectively, to take out their nationals and other foreigners who wanted to leave Lebanon.

The ships were scheduled to arrive tomorrow at the Christian port of Jounieh, 12 miles north of the capital, and sail for Cyprus about 100 miles to the west.

Foreigners as well as West Beirut residents have become increasingly apprehensive about law and order. As the Israeli invasion entered its second week today, fewer and fewer checkpoints were manned by Syrian troops of the Arab Deterrent Force that has had the Arab League's blessing to police much of this country in the wake of the 1975-1976 civil war.

As the Syrians have disappeared from the streets, their place has been taken by well-disciplined Palestinian military police in many key sectors. More than 40 local gangs--several controlling no more than a few city blocks--have also taken to the streets to protect their territory as much as to thwart any Israeli efforts to enter the city.

West Beirut residents, who even in relatively good times do not venture out after dark much, have holed up in their apartments.

Adding to the feeling of insecurity is the lack of electricity in much of the capital as a result of the Israeli destruction of cables from the main power plant at Jiyyeh just north of the port city of Sidon.