Commandos of Yasser Arafat's Al Fatah commando group clashed with radical Palestinians in southern Lebanon yesterday and first reports said at least 12 people had been killed or wounded.

Palestinian sources said the fighting flared Thursday night because of disagreement over the temporary detention of 40 men of the U.N. peacekeeping force in Lebanon by hard-line commandos last Wednesday.

As the Palenstinians battled each other with automatic weapons and rocket-propelled grenades near the southern ports of Sidon and Tyre, panic-stricken civilians fled Tyre for safer areas, witnesses reported.

Fatah threw up 14 roadblocks on a six-mile stretch of the main coastal road reading from Tyre to Beirut, the witnesses said, and encircled pockets of radicals entrenched in the refugee camps of Al Buss and Ain Heleh in Tyre and Sidon respectively.

On Wednesday, guerrillas of the hard-line Rejection Front seized 40 U.N. troops and held them captive for five hours.

No breakdown of the casualty figure for yesterday's fighting was available. But Palestinian sources said several commandos were killed.

Witnesses said it involved commandos of all groups of the Rejection Front with the exception of its leading organization, the Popular Front for the Liberation of Palestine.

A French U.N. officer in Sidon said the U.N. force had been placed on alert. But there was no indication that it was involved in any action.

Thursday night the Palestinian news agency WAFA blamed pro-Iraqi commandos of the Palestinian Liberation Front for the temporary detention of the U.N. troops and said the PLF acted on orders from outside - a clear reference to Iraq.