A Moslem group distributed a videotape tonight of a man hanging from a scaffold and identified him as missing British journalist Alec Collett, a captive since March 1985.

The Revolutionary Organization of Socialist Moslems, a Libyan-inspired underground group, delivered the four-minute tape and a typewritten statement in Arabic to the offices of the Beirut newspaper An Nahar. No body has been found to substantiate the killing, which the group said was carried out in retaliation for the "joint savage American-British raid" on Libya.

Last week, the body of American hostage Peter Kilburn, 61, found alongside those of Britons Philip Padfield and Leigh Douglas, was initially misidentified as that of Collett. A written announcement of Collett's killing on Thursday, the day the three bodies were discovered on a mountain road in the Druze-controlled hills east of Beirut, added to the confusion.

Tonight's videotape showed a swinging noose against a background of green wilderness and rocks while a man's voice read out the communique in Arabic. Then the head of a man with a black blindfold, bearing a resemblance to Collett, appeared on the screen.

With a swollen tongue hanging out of his mouth and his head to one side, the man was lifted with the rope and shown swaying, his hands tied behind his back. Only one hand, with five fingers, was clearly visible. Collett's daughter, Suzie Grant, has said her father had a finger missing. The strong jaw, the receding hairline and rough nose resembled Collett's photographs.

The statement accompanying the videotape was dated Thursday and was identical to one delivered that day to the Beirut offices of an international news agency. In it, the Revolutionary Organization of Socialist Moslems called on all groups still holding American and British hostages to "carry out the death sentence against them, because the blood of Americans and Britons is now lawful everywhere and wherever they may be as a result of the criminal acts of the killer Reagan and the criminal Thatcher."

The man shown in the videotape was wearing jeans, a red and white checked shirt and a tan jacket. As his body swayed, chants of "God is great" were heard in the background, along with shouts of "Down with British and American haughtiness and arrogance, long live the Arab Libyan masses. Down with Reagan the killer and down with the assassin Thatcher."

If Collett's execution is confirmed, he will have been the fourth Briton to be killed by his captors in Lebanon since last year. British teacher Denis Hill was slain following his abduction on May 29, 1985. Douglas and Padfield, also teachers, were slain a week ago.

Collett, 63, a New York-based free-lance journalist, was on temporary assignment in Lebanon for the United Nations Relief and Works Agency when he was abducted.

In New York, U.N. Secretary General Javier Perez de Cuellar expressed "outrage and revulsion" at news of the videotape, saying that Collett had been sent to Beirut "on a humanitarian mission."

The latest killings have driven the few remaining westerners out of Moslem west Beirut. On Sunday, 32 British residents were evacuated to east Beirut in a heavily armed convoy. Yesterday, 10 Americans followed. On Thursday, a similar evacuation of French expatriates is scheduled.

Last week's murders have triggered protests from Lebanese Moslem leaders, outraged by the vulnerability of foreigners to acts of terrorism on Lebanese soil while their protection was assured in radical Arab countries.

Details of how the kidnapings were carried out have started filtering to officials of the American University of Beirut. According to unverified intelligence reports from the extremist organization Hezbollah, Padfield and Douglas, the two British teachers, were kidnaped by a local gang that handed them over to a family from the Bekaa Valley living near Beirut. The reports said the two Britons were moved to the Bekaa and driven in a Libyan diplomatic car to a neighboring Arab country where they were killed.

According to this account, their bodies were brought back and dumped in an area controlled by Druze leader Walid Jumblatt's Progressive Socialist Party militia to embarrass Jumblatt for his overtures for Christian-Druze reconciliation.

Jumblatt and other Lebanese leaders have denounced the campaign against foreigners that has chased teachers and journalists out of Beirut as a plot aimed at depriving the city of its cultural heritage as a meeting place for intellectuals.