Libyan leader Muammar Gaddafi's regime is "clearly crumbing" and NATO will work with Libyan rebels to guarantee a peaceful transition, the head of NATO said on Sunday midnight.

"The Gaddafi regime is clearly crumbling. The sooner Gaddafi realizes that he cannot win the battle against his own people, the better -- so that the Libyan people can be spared further bloodshed and suffering," NATO Secretary General Anders Fogh Rasmussen said in a statement.

Rasmmusen also urged remaining pro-Gaddafi forces to lay down their arms.

"The world is watching them. This is their opportunity to side with the Libyan people and choose the right side of history," he said.

As for the transition following the collapse of the Gaddafi regime, the NATO chief said "the transition must come peacefully. It must come now. And it must be led and defined by the Libyan people."

NATO is ready to work with the Transitional National Council, which must make sure that "the transition is smooth and inclusive, that the country stays united, and that the future is founded on reconciliation," he said.

In addition, Rasmussen urged the international community to provide assistance, with the United Nations and the Contact Group playing a leading role.

According to the latest development, Libyan rebel forces have controlled Tripoli except Bab Al-Aziziyah and are clearing up the remnants of Gaddafi's forces, National Transitional Council's senior official Abdullah Almayhop said.

He said that Gaddafi's eldest son Mohammed has surrendered to the rebels.

Mustafa Abdul Jalil, the head of Libya's National Transitional Council (NTC), said in an TV interview earlier Sunday night that Saif al-Islam, Gaddafi's second son, had been captured and was under custody in a safe place.

Jalil said that Gaddafi's guards in Tripoli had surrendered to the opposition troops.

Meanwhile, Gaddafi said in an Audio speech on Sunday that all the tribes and people should march to Tripoli to "purify" the city, according to the State TV.

"The west will not protect you," he said, "Tripoli will be destroyed."