Troops from the Christian militia units that have been accused of murdering hundreds of men, women and children in Palestinian refugee camps here moved into Israeli-controlled security areas and fraternized with Israeli troops shortly before, and during, the rampage of killing, reliable witnesses said today.

The witnesses, who include diplomats, Lebanese residents of the neighborhoods around the camps, survivors of the attacks and foreign medical workers, acknowledged that it was impossible to identify the particular militiamen they saw as the killers who moved into the Shatila and Sabra camps shortly afterward to begin the massacre of unarmed civilians.

But their accounts, and the discovery of a clearly marked path established for militia convoys through an isolated Israeli-held staging point and then into the area of Shatila, heightened suspicions here of Israeli complicity in the movement of the militia into the camps.

None of the witnesses, nor any other evidence gathered in an effort to reconstruct the events of the past four days, suggest that Israeli troops participated in the slaughter. Israeli officials vehemently deny that their troops even knew it was going on.

But officials in Jerusalem have begun to explain to journalists that their troops did let the right-wing militiamen pass through their lines on the way into the camps as part of an effort to track down any Palestinian guerrillas who had stayed behind in Beirut after the guerrilla evacuation last month. They went on to say that the militiamen went amok, and that Israeli troops intervened as soon as they became aware of the atrocities.

"Nobody dreamed this would happen," United Press International quoted an Israeli official as saying.

But as Red Cross workers and Lebanese Boy Scouts began today to pick up the savaged corpses of the Palestinians caught in the rampage in Shatila and Sabra, new accounts emerged to challenge some of the basic premises of the Israeli professions of initial ignorance and quick response. The witnesses' assertions included the following:

* The massacre was conducted by two separate, Israeli-supported Christian militias -- that of cashiered Lebanese Army Maj. Saad Haddad, Israel's surrogate in southern Lebanon, and members of the late president-elect Bashir Gemayel's Phalangist Lebanese Forces, the dominant militia of Christian East Beirut. A Gemayel family member reportedly acknowledged to a U.S. diplomat that Phalangist militiamen were in the camps during the massacres.

* Israeli Army troops let members of the two militia groups pass through their lines and allowed them to assemble at a part of Beirut's international airport that is under their direct control shortly before the attacks began. The militia convoys then appeared to establish a command post about 300 yards from the entrance to Shatila and immediately across the street from a fully manned Israeli observation post that looked directly down into the camp.

* Journalists, diplomats and Western medical workers saw Israeli troops in the immediate vicinity of the camps at the time the massacres began Friday afternoon, throughout the night and early yesterday morning at the time the killers appeared to have finished their work. Unarmed Palestinian prisoners and in one case a group of European and American medical workers were marched past Israeli troops.

The sighting of Israeli troops casually talking to the militiamen at the entrance to the camp at 7 a.m. yesterday, after most of the killing had stopped, and later of the militiamen withdrawing from the area through Israeli lines with truckloads of Palestinian prisoners, raised questions about Israeli claims to have halted the slayings several hours after they had begun.

Lebanese politicians continued today to make explicit the point that a White House statement strongly suggested yesterday: that a tragic chain of events that led inevitably to the massacre -- as these politicians and others had predicted would happen if the camps were left unprotected -- was begun with Israel's invasion of Moslem West Beirut Wednesday within hours of the assassination of Gemayel, 34, leader of the Christian Maronite Phalangist movement, which Israel had hoped to turn into an open ally in running Lebanon following Israel's June 6 invasion.

Israel said it moved into West Beirut, despite what the Reagan administration said was a clear and binding pledge to special envoy Philip C. Habib not to go into the Moslem-dominated section of the capital, to impose order and avoid any outbreaks of violence. But today, pressed by journalists on why Israel had been unable to prevent the massacre, Israeli Army Chief of Staff, Lt. Gen. Rafael Eitan, said during a press conference at a makeshift command post less than a quarter of a mile from Shatila camp:

"We don't give the Phalangists orders and we are not responsible for them. The Phalangists are Lebanese and Lebanon is theirs and they act as they see fit."

At about 2 a.m. Wednesday, according to Lebanese Army officials, and only several hours after the Lebanese government had officially confirmed Gemayel's death, Israeli Army units began to move north into West Beirut from previously secured and prepared positions in the southern suburb of Bir Hasan, just southwest of the Shatila refugee camp. By noon, Israeli Army units supported by tanks and armored personnel carriers had totally surrounded the camp and the nearby Palestinian neighborhood of Sabra.

According to foreign doctors at Sabra's Gaza Hospital, just north of Shatila, the camp was shelled by Israeli cannons and by mortars that afternoon. By nightfall, the hospital had received 25 wounded and their first reports of deaths in the camp where once up to 10,000 people had lived.

Villagers who live in Shuwayfat, southeast of West Beirut, and along a road from Christian East Beirut -- where the Phalangists are the dominant force -- to the coastal road that leads to southern Lebanon -- where Haddad's troops have been built up by Israeli funding and training -- reported today that at about 3 p.m. Thursday trucks and jeeps marked with Haddad's militia's insignias began pouring into Shuwayfat, to drive along an Israeli Army-controlled road that leads to Israeli military positions around the southern end of the Beirut airport's western runway paralleling the coastline.

Israeli troops seized the airport early in the invasion, and now appear to have moved out of the main terminal area while maintaining control of the coastal runway where small planes land and where Haddad troops were filmed by Israeli cameramen recently.

At the same time Thursday other military transports were coming down from East Beirut and turning into the same airport road, which was marked distinctly by a new sign post that bore the Phalangist Lebanese Forces' insignia, a triangle inside a circle, along with the letters "MP" and an arrow pointing down the road, the residents said.

I located the turn-off sign today. Immediately beside it in the airport perimeter was a sign in Hebrew lettering with an arrow pointing in the same direction, into the Israeli staging area. An Israeli Army jeep emerged from that area as I watched.

Similar signs, which could be used to direct troop movements and convoys, were spaced every 50 yards, sometimes spray painted on a wall or on signposts standing by the road.

Two hours after the convoys were seen going into the southern end of the airport Thursday, an estimated two battalions, or about 1,600 men wearing Christian militia uniforms, were seen moving from the direction of the airport into Bir Hasan, next to the Kuwaiti Embassy crossroads. This brought them immediately adjacent to the Israeli Army operational headquarters in the area, located around three bombed-out, seven-story apartment buildings that had once served as officers' housing for the Lebanese Army.

The Christian militias drove to Bir Hasan along the airport road, turned left to drive toward the sea from the village of Ouzai, then north into Bir Hasan where they set up their own command post in the business administration building of the Lebanese University, directly across from an Israeli command post. To get there, the militias followed the arrows of the road markers of the Lebanese Forces that were spray-painted all the way to Bir Hasan, along the route retraced today by this correspondent.

Several hours after that--while Israeli tanks continued to occupy positions with their cannons pointing into the camp they had shelled previously -- the militiamen entered Shatila camp through its southern gate about 300 yards east of the Israeli and Christian command headquarters at Bir Hasan.

At about 7 p.m., European doctors working at Gaza Hospital, started hearing the rattle of small- arms fire, the explosion of grenades and mortars, and the first wave of casualties and terrified refugee families started flowing in. The sky over Shatila was lit through most of the night by flares fired repeatedly into the sky, either by the Israelis or the Christian militia.

With the ghostly light of those flares streaming in the hospital windows, the hospital's medical team, which included 20 foreign doctors and nurses, worked through the night. By dawn, the hospital had admitted 82 people wounded by rifles or shrapnel, and the hospital was jammed with 2,000 refugees from Shatila.

Sometime Friday morning, local residents around the Shatila camp, saw the Christian militiamen march out a group of about 100 men from the camp onto the four-lane road that forms its southern border. Lebanese were herded to one side and forced to sit down, while Palestinians were grouped on another side.

A 15-year-old Palestinian, whose story was corroborated by half a dozen other people in the neighborhood, said he saw at least half the Palestinians were bleeding from their faces.

"They were standing over them with knives and asking them questions," said the boy, whose name cannot be given, as other people from the area nodded in assent. "Those that did not answer them were cut in the face."

The Palestinians were later led off toward Bir Hasan while the Lebanese men were released.

Later that same morning, according to European medical personnel who later helped evacuate Akka Hospital, just outside the southern entry of the Shatila camp, the Christian militiamen went around the street blowing up buildings on Shatila's southern edge, including a mosque whose conical green dome settled on top of a mound of concrete rubble to look like a scoop of pistachio ice cream.

At one point, the militiamen entered the hospital, shot two Palestinian doctors and a civilian, and took the three wounded men away. Later they returned, and on the hospital premises, repeatedly raped a young Palestinian nurse named Intisar Ismail then shot her dead, witnesses said.

When a team of four Palestinian doctors and nurses walked out of the hospital under a white flag to try to tell the militiamen not to shoot at the hospital, a militiaman threw a hand grenade that killed three of them, and wounded the fourth. The hospital staff, and five wounded patients, were evacuated by the international Red Cross thereafter.

At 11 a.m. Friday I managed to find a route through a ring of Israeli Army roadblocks that cut off most of West Beirut from the camp area. The area was strangely silent, but Christian militiamen were manning a roadblock in front of the Kuwaiti Embassy, at the southwestern limits of the camp, and Israeli soldiers were everywhere, lounging around armored personnel carriers and tanks whose cannons still pointed into the camp.

A group of 12 young Palestinians were seen being led down the Camille Chamoun Boulevard that marks the camp's far western edge, led by a Lebanese Forces soldier. They marched past the Israelis and were taken to the militia command post, at the business school of Lebanese University.

Two hours later, militiamen wearing arm patches of the Lebanese Forces, turned me away from the camp gate.

At 4 p.m. a reporter for an American newsmagazine reached the Kuwaiti Embassy crossroads. There he talked with a Christian militia officer, who said he had just brought his men out of the camp for a "bit of rest" before going back in. Israeli soldiers were lounging all about them, some reading calmly despite the rattle of gunfire in the camp. The gunfire was in short bursts, in roughly one place, and did not indicate that there was any return fire. The Christian militiamen were being fed and given water by the Israeli Army.

U.S. diplomats spoke to Amin Gemayel, the brother of the slain president-elect, by telephone at about 4 p.m. and asked if any Phalangist units were in Shatila. Amin Gemayel, who is now the Phalangist candidate for the presidency, acknowledged that his militiamen were present, but he said they would be leaving within an hour, diplomatic sources reported.

At about 6 p.m., this correspondent once more reached the edge of the camp, in the company of a Western diplomat. Men wearing Phalangist Lebanese Forces uniforms were still in the area. At the militiamen's headquarters in the business school, Israeli officers as well as Lebanese Forces officers were seen together, standing and talking before a group of jeeps, all with the markings of the Lebanese Forces.

There was no roadblock at the Kuwaiti Embassy this time and no guards at the camp entrance. But after we had driven about 150 yards into the camp, a group of militiamen, some with patches of Saad Haddad's southern militia, others with those of the Lebanese Forces halted the car and ordered it to leave. Several fires were burning in the camp, but, at that time, there was a great stillness.

At 6 a.m. yesterday Christian militiamen arrived at Gaza hospital and ordered everyone into the street on the threat of storming the facility.

The Palestinians and Lebanese were led off first, then the foreign staff of 20 doctors and nurses, two of them from the United States -- Ellen Siegel, of the Washington Hospital Center, and Dr. Michael Knipmeyer. They were all herded southward down the main street of Shatila.

At the camp entrance, which the group reached at about 7 a.m., a group of 100 militiamen were entering with their rifles at the ready. The foreign medical group was marched up to the business school in clear sight of the Israeli Army units still in positions all around them then taken around to the back of a building for interrogation.

The medical team was later turned over to the Israeli Army officers across the street.

By 9 a.m. yesterday, when the first foreign correspondent entered the refugee camp, he was greeted with stillness and death.

A Western diplomat ordered by his embassy to try to make a count of the dead today said he counted 106 bodies in the camp although many were still hidden in concrete buildings that had been demolished by dynamite sometime early yesterday morning. Parts of bodies were protruding from piles of rubble pushed up by bulldozers. It was not known how many people might be in a mass grave under Israeli Army observation, where the bodies of two men were found partially buried on the edge of a pit which showed signs of having been freshly covered with red earth.