The Palestine Liberation Organization and Israel ordered their forces tonight to observe a cease-fire following a day of the most intensive bombing of the Lebanese capital since Israel invaded Lebanon last Sunday.

Israel first announced that it was extending its day-old cease-fire with Syrian units in Lebanon to the Palestinians. Details on Page A24. Shortly afterward, PLO leader Yasser Arafat also ordered his troops and those of allied leftist Lebanese forces to halt fighting tonight.

Both announcements said the cease-fire would take effect at 9 p.m. local time 3 p.m. EDT . Fighting appeared to stop almost immediately, and Beirut appeared to be quiet four hours after the deadline.

Israeli warplanes bombed Beirut and its outskirts for nearly 12 hours today, killing at least 200 persons, according to Palestinian accounts. While Israel did not explain its decision to halt the fighting, the rising casualty toll inflicted on the Palestinians was clearly a factor in Arafat's decision.

In a declaration given to the United Nations office here, Arafat said the PLO was accepting the cease-fire on the basis of U.N. Security Council resolutions adopted last week calling for immediate cessation of hostilities and unconditional Israeli withdrawal to its border.

"This is done in response to the Arab and international efforts in that regard," Arafat said in his message handed to Samir Sanbar, the chief U.N. spokesman in Lebanon.

As with yesterday's cease-fire with Syria, Israel did not mention a withdrawal. Israeli Foreign Minister Yitzhak Shamir, although he had spent the day in meetings with U.S. special envoy Philip C. Habib, said his country's decision was unilateral and not the result of "contacts with outside elements."

A State Department spokesman said in Washington that the United States welcomed the cease-fire and hoped that both sides would not "undertake any action to renew the fighting."

Israel invaded Lebanon last Sunday with the announced intention of clearing a 25-mile-deep guerrilla-free zone along its border and then carried its warfare farther north and into Syrian-held territory. Before today's cease-fire, Palestinians reported that Israeli forces began their bombing of Beirut and nearby areas..

The bombing by waves of warplanes and shelling by gunboats off the Mediterranean Coast lasted 12 hours and leveled buildings at Palestinian refugee camps south of Beirut and destroyed or damaged four civilian airliners at Beirut International Airport. Rockets caused extensive damage to the palace of Lebanese President Elias Sarkis in suburban Baabda. Sarkis and U.S. Ambassador Robert Dillon had met there two hours before, but both were gone by the time the rockets hit.

Another rocket landed just a block from the Soviet Embassy in West Beirut, the largely Moslem and Palestinian area that was the main target of the attacks.

Sources close to the PLO leadership said the diplomatic contacts that led to today's cease-fire were carried out primarily at the United Nations. A Western diplomatic source said U.N. Secretary General Javier Perez de Cuellar had been particularly active in working out an end to the bloodshed. Arafat was said to be in touch yesterday and today with Assistant Secretary General Brian Urquhart through the U.N. office here.

The cease-fire came 34 hours after the one agreed to by Syria and Israel. Western intelligence sources in touch with their Lebanese counterparts said, however, that Israeli air strikes at midday against units of a Syrian brigade on the western portion of the Beirut-to-Damascus road, had taken a heavy toll. There was no independent confirmation of Israeli strikes against Syrians.

The intelligence sources said that at the time of the cease-fire Israeli forces were positioned two miles south of the Beirut-to-Damascus highway along the entire Bekaa Valley. Realizing their untenable position, the sources said, the remaining Syrian units in the strategic Bekaa Valley had withdrawn to Baalbek, 20 miles north of the highway, and beyond.

The Lebanese Army had been reported worried lest the Israelis slice across the Beirut-to-Damascus road and race northward to capture the port city of Tripoli, joining up with their former allies, the Christian Maronite militia commanded by Bachir Gemayel.

Just south of Beirut Airport, Israeli air, land and sea attacks from before dawn to after dusk--and before the cease-fire--concentrated on the so-called "Khalde triangle" where Palestinian, Syrian and Lebanese militias are massed in defense of the capital. The triangle stretches from a vital seaside road junction leading down the coast into Beirut and up into the mountains, eventually linking up with the highway to Damascus.

Israeli armor was reported in control of the mountain crossroads leading down to the sea and up to the Druze city of Aley, but Palestinian spokesman Bassam Abu Sharif insisted the guerrillas and their Lebanese allies, including the Amal Shiite forces, which had been at odds with the Palestinians before the invasion, had stopped the Israelis at a road junction. He said their forces were counterattacking.

"We will keep defending that triangle to keep the Israelis from penetrating the outskirts of Beirut," he said. Other Palestinian sources admitted that their troops had been taking a beating for the past four days but said they were holding fast thanks to the zigzagging road network and heavy tree cover.

There was no clear casualty count from the most recent fighting but the International Committee of the Red Cross, quoting Lebanese Red Cross sources, estimated the death toll this week among civilians in Sidon, 25 miles south of Beirut, at "between 1,000 and 1,200." They said 3,000 were wounded in the coastal city, Lebanon's third-largest, which was captured by Israel earlier in the week. A Red Cross delegate said the death toll was only an approximation since "bodies were being laid out by tens in a parking lot" and counted by rough estimation.

Another Red Cross delegate estimated that the number of displaced persons had reached 600,000, more than twice the number displaced when Israel invaded a smaller portion of southern Lebanon in 1978.

Francesco Noseda, of the International Red Cross Committee, said that a survey showed 90 percent of the three Palestinian camps around Tyre, between Sidon and the Israeli border, had been destroyed.

Noseda also said Sidon hospitals were "just barely functioning" and that the food, health, shelter and santitation conditions along the entire coastal strip devastated by the Israeli invaders remained "very, very bad and precarious."

The Red Cross has a cargo plane waiting in Cyprus to fly in 10 tons of medical supplies, clothing and tents, when Beirut Airport conditions improve. Two larger planes are on standby in Switzerland.

Refugees were said to be fleeing in all directions--some from Sidon to the inland town of Jazzin, whose normal population has risen tenfold.

Here in Beirut, reports in the French-language newspaper L'Orient-Le Jour put the death toll from Israeli bombing and shelling yesterday at 207 deed and 647 wounded. A Palestinian spokesman said "no less than 200" civilians were killed in today's attacks.

Abu Sharif accused the United States of being a "direct partner" in the "mass killings," charging that it had given the "green light and full support" for the invasion.

The handful of diplomats remaining in the U.S. Embassy, after a partial evacuation earlier in the week, are fearful for their lives, according to a well-informed diplomat, and feel isolated and cut off from information from Washington.

Whatever their inner fears, Palestinian officials, before the announcement of the cease-fire, were putting a brave face on what they thought would be a major Israeli assault on their fighters.

One official said, "for years we've been desperately trying to get one man across into Israel at a time and now they are doing us the favor of coming to us."

"Beirut is ideal guerrilla fighting territory," he said.

With the southern outskirts virtually deserted of Palestinian and Moslem Lebanese, the northern end of West Beirut is teeming with refugees. Families are camping out in lobbies, steps and staircases of buildings and occupying apartments, sometimes by force. Elsewhere refugees have been provided shelter in schools and mosques.