"With all sadness, I inform you of the passing of Abdel Fatah Younes, the commander-in-chief of our rebel forces," NTC chief Mustafa Abdel Jalil said late.
"The person who carried out the assassination was captured," Abdel Jalil said without elaborating.
Younes was shot and killed by an armed group while he was on his way to Benghazi after he had been called in from the front to answer questions over the military situation, Abdel Jalil said. He said there would be three days of mourning in Younes's honour.
Rumours circulated in Benghazi earlier yesterday that Younes, known as the number two in Gaddafi's regime prior to his defection in the early days of Libya''s revolt, was arrested but they could not be confirmed by AFP.
Abdel Jalil said he was giving a final warning to armed groups to either join the rebel armed forces on the front or NTC security forces in the cities.
Moments after the announcement, two vehicles loaded with an anti-aircraft gun and at least a dozen armed men came into the driveway of the Tibesti hotel, where the announcement had been made, an AFP reporter witnessed.
A witness said that they managed to enter the hotel with their weapons but security forces calmed them down and convinced them to leave.
"They shouted ''You killed him,' in reference to the NTC, he added.