TRIPOLI, Libya — Akram al-Warfalli, a leader of this country’s giant Warfalla tribe, said only a few words in his interview with Al Jazeera: “We tell the brother Qaddafi, well, he is no longer a brother,” Mr. Warfalli said. “We tell him to leave the country.”

It appeared to be the defection of a powerful tribe that has supported Col. Muammar el-Qaddafi — a powerful inspiration for the revolts that were then taking shape in the east. Some Libyans and scholars outside the country say the system of tribal alliances that has long been Colonel Qaddafi’s most potent weapon is now emerging as perhaps a potential vulnerability, as external wars, failed tribal coups and internal purges have chipped away at his support, even among his own tribe and its allies.

Colonel Qaddafi’s reaction to the tribe’s mutiny was swift, dispatching fighting units to the Warfalla’s traditional homeland of Bani Walid, another Warfalla leader recalled Monday. There they made sure no younger tribe members left to join uprisings in the nearby cities of Zawiyah and Misurata, as well as here in the capital, this Warfalla leader said, speaking on condition of anonymity for fear of retribution against himself and his family....