BEIRUT, Lebanon (AP) _ A terrorist group that claims it hanged a kidnapped British journalist urged other extremist groups holding British and American hostages to kill them in revenge for the U.S. air raids on Libya.

Seven French teachers left Moslem west Beirut today for the city's Christian sector, fearing violence by Moslem extremists. Eight Americans, who fled west Beirut on Tuesday, left Lebanon today by boat for the Mediterranean island of Cyprus.

A four-minute videotape showing a blindfolded man dangling from a scaffold was left Wednesday at the independent Beirut newspaper An-Nahar in the name of the Revolutionary Organization of Socialist Moslems.

A typewritten statement left with the videotape identified the man as 64- year-old British journalist Alec Collett. A voice on the videotape said in Arabic that Collett was hanged at a mass rally, but it did not say where.

Collett had a finger removed in surgery several years ago, but it could not be seen from the videotape whether the corpse was missing any fingers.

''They can't see clearly enough in the videotape of the hands to determine whether it is him,'' Collett's son, David, said Wednesday in a radio interview in San Fernando, Calif. ''However, from the report, it sounds very much like him.''

Collett's body has not been found. In London, the British Foreign Office said it could not confirm he had been killed. ''Until the facts have been established beyond doubt, speculation will only cause further pain and anguish to Mr. Collett's family, friends and colleagues who have already suffered so much distress,'' said a Foreign Office spokesman, who by custom spoke on condition of anonymity.

The statement purported to be from Collett's kidnappers said he was killed April 16 in retaliation for Britain's decision to allow the United States to use bombers based in Britain to attack Libya.

It urged other groups holding American or British hostages to ''execute them because the blood of Americans and Britons is now permissible as a result of the criminal acts exercised by murderer (President) Reagan and criminal (British Prime Minister Margaret) Thatcher.''

Five Americans, one Briton and a man with dual British-Irish citizenship are among those missing in Lebanon.

Last week two kidnapped Britons and a kidnapped American were found slain outside Beirut. A note said they were killed in retaliation for the raid on Libya, which Reagan ordered because of alleged Libyan involvement in international terrorism.

The bodies were identified as those of American Peter Kilburn, 62; and Britons Leigh Douglas, 34, and Philip Padfield, 40.

An official at the American University Hospital morgue said today that the bodies of Douglas and Padfield have been cremated and the ashes were sent to the British Embassy in east Beirut. The embassy arranged a memorial service for the two men at the Evangelical Church in the Christian suburb of Rabieh.

Kilburn's body has been flown back to the United States for burial.

A Western diplomat, who spoke on condition of anonymity, said fewer than 65 foreigners remained in the city's Moslem sector, which bustled with thousands of Westerners before Lebanon's civil war began in 1975.

Frenchman Alain Lachner, a teacher at west Beirut's Lycee secondary school, said today he was leaving the city's Moslem sector because of the slayings of Padfield and Douglas.

''Collett's death did not affect me as much as that of my two British friends. I spent some vacations with them,'' Lancher said.

''It hurts. I have ties, I have friends,'' he said of leaving west Beirut. ''I don't know what I'm going to do once I'm in France.''

He was among the six men and one woman who crossed into Christian east Beirut today under the protection of French Embassy guards. The U.S. and British embassies have evacuated their nationals from west Beirut in recent days, with eight of the Americans and some of the Britons leaving Lebanon altogether.

Collett was kidnapped in south Lebanon on March 25, 1985 while on a writing assignment for the U.N. Relief and Works Agency, which serves Palestinian refugees.

The group that claims it killed Collett is believed associated with renegade Palestinian leader Sabry al-Banna, code-named Abu Nidal. The slayings of Kilburn, Padfield and Douglas were claimed by the Arab Revolutionary Cells, another group believed linked to Abu Nidal.

Islamic Jihad, or Islamic Holy War, believed made up of Shiite Moslems loyal to Iran, has said it abducted the five missing Americans.

They are Terry A. Anderson, chief Middle East correspondent of The Associated Press; the Rev. Lawrence Jenco, a Roman Catholic priest; David Jacobsen, director of the American University Hospital; Thomas Sutherland, acting dean of the university's school of agriculture; and U.S. Embassy political officer William Buckley.

Islamic Jihad claimed Oct. 4 that it killed Buckley, but no body has been found.

British cameraman John McCarthy was kidnapped April 15. English teacher Brian Keenan, who holds both Irish and British citizenship, disappeared April 11.