When the revolt broke out it was Seif al-Islam who delivered the government’s first public response, vowing to wipe out what he called “the rats” and warning of a civil war.

In his last public interview, he appeared a changed man. Sitting in a spare hotel conference room, he wore a newly grown beard and fingered prayer beads. After months of denouncing the rebels as dangerous Islamic radicals, he said that he was brokering a new alliance with the Islamist faction among the rebels to drive out the liberals.

While rebels expressed hope that Colonel Qaddafi’s forces had lost their will to fight, support for the government could remain strong inside areas of Tripoli. Analysts said the crucial role played by NATO in aiding the rebel advance in the relatively unpopulated areas outside the capital could prove far less effective in an urban setting, where concerns about civilian casualties could hamper the alliance’s ability to focus on government troops.

A senior American military officer who has been following the developments closely, and who has been in contact with African and Arab military leaders in recent days, expressed caution on Sunday about the prospects for Libya even if the Qaddafi government should fall. Even if Colonel Qaddafi is deposed in some way, the senior officer said, there was still no clear plan for a political succession or for maintaining security in the country.

“The leaders I’ve talked to do not have a clear understanding how this will all play out,” said the officer, who spoke on condition of anonymity because he was not authorized to discuss the issue publicly.

Few would have predicted that the rebels would meet so little resistance from the 32nd Brigade, a unit that NATO had considered one of the most elite in Libya and commanded by Khamis Qaddafi, one of the leader’s sons. The so-called Khamis Brigade was one of the crucial units enforcing the defense lines around the capital, extending about 17 miles outside Tripoli to the west and about 20 miles to the south.

Rebels said those points had been breached by Sunday afternoon despite the expectation that Colonel Qaddafi would use armored units and artillery to defend them. It was unclear whether the government troops had staged a tactical retreat or had been dislodged by NATO strikes.