Israel's single-minded determination to destroy the last remnants of the Palestine Liberation Organization in Beirut has led it into tragic complicity in the bloody massacre of hundreds of Palestinian refugees by its Lebanese Christian allies.

As the details of the event of what one official called "really a black Friday" became clearer, senior officials confirmed that Israeli Army commanders in the area knew the Phalangist forces had entered the Shatila and Sabra refugee areas but stood by, apparently for hours, on the assumption their allies were rooting out of any remaining guerrillas and not killing indiscriminately.

The degree of Israeli involvement remained unclear here today and it was not known whether ranking military and civilian government officials were aware of the Phalangists' entry into the refugee areas before the mass killings began. Senior officials denied reports that there was active "coordination" between the Israeli Army and the Phalangist militia entering the area.

"It was not a case that they were going through our checkpoints and we were waving them through," one official said.

But the official conceded that as early as 2 p.m. Friday Israeli government authorities were informed that "something is wrong" around the Shatila and Sabra neighborhoods in Beirut's southern outskirts. It was about then, in daylight with Israeli Army units ringing the area, that the Phalangist militia began moving in on the refugee centers, he said.

Israeli officials tonight strongly denied persistent reports from eyewitnesses in Beirut that the massacre was largely the work of forces commanded by an even closer ally, former Lebanese Army Maj. Saad Haddad. They insisted Haddad's forces were not in Beirut at the time.

A senior official with access to intelligence information said, "I know that the Phalangists went in, and I know that the Phalangists and Haddad's men don't get on. It is just inconceivable to me that they could have gone in together."

The official said that some of Haddad's militia tried to enter West Beirut earlier last week but were turned back by Israeli forces. The British Broadcasting Corp., however, reported today from Jerusalem that an Israeli cameraman had filmed Haddad's units in Israeli-controlled areas earlier in the week.

During the Israeli siege of Beirut before the evacuation of the PLO, some of Haddad's militia was allowed into the Beirut area by Israeli authorities, according to this official, to "allow them the honor of taking part in Beirut." Asked if the Israelis could not have brought Haddad's forces back to clean out the camps, he said, "This is not only not true, but we prevented them from coming in."

He suggested that the Phalangists put on uniforms of Haddad's men "in a trick that is as old as the Bible."

Any role in the massacre by the Haddad forces would indicate strong Israeli complicity in the incident. Haddad with his Israeli-sponsored and -supplied militia has controlled a strip of land along Lebanon's southern border with Israel where his troops served as proxies for the Israelis in fighting the Palestinian guerrillas. But he has denied any role in the Beirut massacre.

Officials here argued vehemently that once the Israeli Army realized a massacre was taking place it moved quickly to oust the Phalangists from the area. However, those arguments follow previous Israeli claims that they seized military control of Beirut to prevent such a bloody incident.

In the days leading to the massacre, it became increasingly clear here that Israel's purpose in ordering its army into West Beirut was less the preservation of order following the assassination of president-elect Bashir Gemayel than it was the rooting out any remaining guerrillas in the city.

That always has been the intention of the government of Prime Minister Menachem Begin. It went to war in June to drive the PLO away from Israel's northern border with Lebanon, but the Israelis did not stop until they reached the gates of Beirut, trapping the guerrillas inside the city. There were even reports during the initial phases of the Israeli invasion in June that Defense Minister Ariel Sharon wanted to send the Phalangist militias into the camps to eradicate the PLO. But the plan was apparently foiled because the Phalangists were either not eager to take the job or believed they were not strong enough to take it on.

Even after the evacuation of Palestinian fighters, however, the Israelis insisted that guerrillas remained behind in Beirut. The Israelis preferred that the job of disarming any guerrillas be taken on by the Lebanese Army. According to a well-informed senior official, at a meeting here Friday with U.S. envoy Morris Draper, Sharon and Foreign Minister Yitzhak Shamir strongly urged that the Lebanese Army enter the refugee areas, "the sooner the better."

Later that day, the official said, Israeli military authorities discussed just such a move with a military representative of Lebanese Prime Minister Shafiq Wazzan. But according to the Israeli official, the Lebanese said they needed authorization from the government and pointed out that it was nearing nightfall and that they did not intend to order troops into the area in darkness. "Friday, the critical day, was wasted," the Israeli official said.