Israel continued its war unabated against Palestinian guerrillas today, pounding their positions around the capital with air strikes, artillery fire and naval bombardments despite a cease-fire between Israeli and Syrian forces that began at noon inside Lebanon.

Israeli warships off the coast of Beirut lobbed shells indiscriminately into the center of the city until early evening, killing at least 20 persons and wounding many others.

Neither Israel nor the Palestine Liberation Organization seemed ready to end their fighting, but there was a flurry of confusing statements from guerrilla leaders that seemed to indicate they would agree to negotiate a cease-fire on certain terms.

After a long meeting of the top PLO leadership, a leading official, Faleh Khalif, also known as Abu Iyad, was quoted by the Palestinian radio station as saying: "We have not been consulted about it, so we are not concerned by the cease-fire in any way."

But the PLO's top leader, Yasser Arafat, sent a message to U.N. Secretary General Javier Perez de Cuellar saying he accepted Security Council resolutions 508 and 509 calling for a cease-fire and an unconditional Israeli withdrawal.

The PLO deputy U.N. observer, Hasan Abdel Rahman, said the PLO could not cease hostilities as long as Israel defied the Security Council demand for a withdrawal from Lebanon, the Associated Press reported from U.N. headquarters.

Earlier, a PLO spokesman told a news conference here, "We will continue to attack them as long as they are on our territory."

It was not immediately clear from the statements whether the PLO was rejecting outright the idea of the halt to hostilities or simply asking, as Khalif seemed to be hinting, that it be consulted about a separate cease-fire before accepting it.

A later comment by a PLO source, quoted anonymously by Reuter, seemed to indicate that the guerrillas were negotiating for assurances Israeli forces would not take advantage of a cease-fire. He said the PLO would abide by one if the Israelis did not reinforce their positions or push forward against the guerrillas.

Last July, after two weeks of fighting between Israel and the Palestinian guerrillas, a cease-fire was arranged through the joint mediation of Saudi Arabia and the United States. Whether this will happen again this time was far from clear.

Western diplomats here said the Reagan administration had been anxious to bring a halt to the rapidly expanding fighting between Israeli and Syrian forces inside Lebanon for fear of it producing a larger Middle East conflict possibly involving the Soviet Union.

Moscow is bound by a treaty of friendship and cooperation with Syria to help defend Syria from outside aggression.

But the same sources said Washington had shown no similar concern about the Israelis' bid to crush the Palestinian guerrilla movement inside Lebanon since the Israeli invasion began six days ago.

Thus it was felt here that Washington might be far more lenient in pressing Tel Aviv to end its war against the guerrillas. Events on the ground today around Beirut and elsewhere in the country supported that feeling.

So far as could be determined, Syrian and Israeli forces were generally respecting the cease-fire accord. But Beirut radio reported tonight that Israeli artillery in the Chouf region of the Lebanese mountain range shelled Syrian positions along the strategic Damascus highway late this afternoon. In addition reporters saw Syrian and Israeli tanks and artillery exchanging fire near the Israeli beachhead closest to Beirut, just four miles south of the city.

This reporter visited the Chouf town of Ain Dara, where Syrian tanks and artillery had held off an advancing Israeli armored column for four days. There both sides were respecting the cease-fire late in the afternoon.

But fighting between Israeli and Palestinian forces seemed to be continuing around Beirut at about the same pace as before the cease-fire.

In the last hour before it took effect, Israeli jets engaged in a massive attack on Palestinian and Lebanese leftist positions around the southern and western fringes of the city. The warplanes' rockets scored direct hits on the building in west central Beirut housing the military operations center of Arafat's Fatah organization and the apartment of another top leader, Abu Jihad.

The six-story building collapsed under the Israeli rocketing, and two other nearby buildings were also hit. Beirut radio said five people were killed and 82 wounded, mostly civilians. Some of Fatah officers were reported trapped but still alive under the rubble in the basement military center.

A PLO officer said no high ranking PLO official was killed.

Elsewhere in Beirut, Israeli shells, fired either from warships or artillery from the city's outskirts, hit the FM radio antenna of Beirut radio in the upper-class Sakiet Janzir section, killing at least 12 people, including a number of children on an adjacent playground.

Late in the afternoon, shells seemingly fired aimlessly from warships into the city center hit an apartment building, destroying the top three floors. Casualties were not immediately known.

A bomb also badly damaged the apartment building where New York Times correspondent Thomas Friedman lives. At least 10 people were reported killed in the explosion, including the wife and two daughters of Friedman's local assistant. Friedman was away, and his wife is not presently in the country.

Unconfirmed reports later said two groups of war refugees that had occupied the building were fighting among themselves and that the losing side may have decided to get even by blowing up the winners.

The report is symbolic of the breakdown in the little remaining law and order here as refugees pour into the city, and street gangs take over entire blocks in the name of the general resistance to the invaders.

Throughout the day, Israeli artillery and boat fire blasted Palestinian, leftist Lebanese and Shiite Moslem positions around the strategic junction at Khalde, which is at the southern edge of Beirut's international airport, and on the mountain slopes above.

The Israeli action seemed to indicate the invaders were still trying to strengthen their hold on the southern outskirts, where they have established a small beachhead and where many Palestinian refugee camps and military sites are located.

Late tonight Israeli planes were dropping flares over the city, apparently to light up potential targets on the southern outskirts.