A three-truck convoy that left the Red Cross warehouse here laden with emergency food supplies returned still loaded early this afternoon, after being refused passage through the Israeli Army port checkpoint in Beirut, 14 miles to the south.

The warehouse, a humid, underground car garage of the Espace 2000 office building, is stacked ceiling high with tons of Red Cross food, canned water and other emergency supplies.

Thousands of West Beirut residents reportedly gathered to protest the blockade of food and supplies. In another development, Palestinian leader Yasser Arafat offered to disperse his forces temporarily in other parts of Lebanon until a more permanent refuge could be found. Details on Page A10.

The food began arriving at Kaslik, a southern coastal suburb of the port city of Juniyah, by boat from Cyprus weeks ago. It was blocked from shipment into West Beirut through Israeli military checkpoints in East Beirut, however, when Israeli soldiers took over the city's three crossing points between its two halves two weeks ago.

A shipment of food finally was allowed to cross last week, before the beginning of a weekend of heavy shelling. On Monday, Israeli Army spokesman Col. Paul Kedar announced that the blockade was lifted, and Red Cross food convoys would be allowed to cross the Beirut port checkpoint on a "prearranged schedule." But an unscheduled visit to the Red Cross depot at 1:30 p.m. today coincided with the return of the convoy that had been refused passage through Beirut's port.

One Red Cross worker said the flour the convoy had tried to deliver could spoil if kept in the garage much longer. There were signs of deterioration in the soaked cardboard boxes of cooking oil near the front of the garage.

The worker said that Israeli checkpoint policy was erratic and that Red Cross workers were never sure when a convoy would be allowed into West Beirut. At that point, another Red Cross worker angrily told him not to talk to a reporter because "we are forbidden to talk to the press."

Reached by telephone, a secretary at Red Cross headquarters in West Beirut said spokesman Jean-Jacques Kurz and other officials were not available for comment.

Kedar left Beirut this morning for his routine weekend rest in Israel, but another Army spokesman, Michael Kevehazi, said he did not understand why the Red Cross convoy had been turned back.

"There is a definite order that they should be allowed through," Kevehazi said. "It's just another ---- up. It is pure, temporary nonsense that will eventually be cleared up."

Checkpoint policy for crossing between the city's two sections has been vague as the Israelis have stepped up pressure on the estimated 5,000 to 6,000 Palestine Liberation Organization guerrillas in West Beirut.

When the Israelis began to tighten access to West Beirut two weeks ago, there was constant friction at the Israelis' daily military briefings between foreign reporters and Kedar about charges that Israeli soldiers had cut off water and electricity to West Beirut.

Kedar said repeatedly that the shut-off "is not official Israeli policy," but it continued. "There is some confusion about this," was all Kedar would allow at his press conference one week ago.

The Israelis also have closed and reopened the two checkpoints they are now controlling--without warning or explanation. Amid press reports that Israeli Prime Minister Menachem Begin had received a sharp rebuke from President Reagan on Israeli actions toward West Beirut, Kedar announced on Monday that water and electricity were being restored to West Beirut.

Today's blockage of the convoy came amid a growing exodus of Lebanese civilians from Israeli-encircled West Beirut into what is considered the safer haven of the eastern part of the capital.

Lebanese civilians said the growing number of people leaving West Beirut were doing so out of a fear of an eventual Israeli military attack on the strongholds of the Palestine Liberation Organization.

Although the exodus has not become a flood, it is evident in East Beirut that the number of persons crossing over is steadily rising as Sunday's cease-fire, with minor violations, continues to hold.

Lebanese officials say that the number of civilians in the western portion of the city is down from 600,000 to about 300,000 or 400,000.

At midafternoon today, there was a huge traffic jam of cars and trucks, packed with people, furniture and personal belongings, crossing into East Beirut at the Galerie Semaan crossing point. Galerie Semaan is the only checkpoint from which Israeli soldiers have been withdrawn and is being policed now by soldiers from the Lebanese Army.

At the two Israeli-controlled checkpoints, traffic was comparatively light. There was a steady line of cars filtering through the barricades at the crossing near the national museum and only a few cars and small trucks coming through the port.

"There are still a lot of people in West Beirut," said Lebanese taxi driver Nicolas Petro. "The people are afraid to leave their shops and homes because they're afraid they will be looted. They fear they will lose everything."

"I have relatives in West Beirut, and we're unsure if we should go and get them and bring them over here" to the east side, he added. "Some people say the war is finished, there will be no more fighting and others say, 'no the real fight is yet to come.' I don't know. "I will see tomorrow."