Libyan rebels raced into Tripoli Sunday and met little resistance as Muammar al-Qaddafi's defenders melted away and his 42-year rule rapidly crumbled. The euphoric fighters celebrated with residents of the capital in Green Square, the symbolic heart of the regime.

By morning, a rebel spokesman told Al Jazeera that Qaddafi loyalists only controlled between 15 and 20 percent of the city, Reuters reported.

Associated Press reporters with the rebels said they reached the Tripoli suburb of Janzour around nightfall Sunday. They were greeted by civilians lining the streets and waving rebel flags.

Qaddafi's son, Saif al-Islam Qaddafi, was captured by rebel forces as they overtook the city. Al-Islam faces charges of crimes against humanity at the International Criminal Court in the Netherlands. Another son was under house arrest.

"It's over, frizz-head," chanted hundreds of jubilant men and women massed in Green Square, using a mocking nickname of the curly-haired Qaddafi. The revelers fired shots in the air, clapped and waved the rebels' tricolor flag. Some set fire to the green flag of Qaddafi's regime and shot holes in a poster with the leader's image.

By the early hours of Monday, rebels controlled large parts of the capital. They set up checkpoints alongside residents -- many of them secretly armed by rebel smugglers in recent weeks. But pockets of pro-Qaddafi fighters remained: In one area, Associated Press reporters with the rebels were stopped and told to take a different route because of regime snipers nearby.

"We were waiting for the signal and it happened," Nour Eddin Shatouni said. Shatouni is a 50-year-old engineer who was among the residents who flowed out of their homes to join the celebrations.

"All mosques chanted 'God is great' all at once. We smelled a good scent, it is the smell of victory. We know it is the time."

Hours earlier, the same rebel force of hundreds drove out elite forces led by Qaddafi's son in a brief gunbattle. The fighters hauled off truckloads of weapons and advanced full speed toward the capital.

When rebels reached the gates of Tripoli, the special battalion entrusted by Qaddafi with guarding the capital promptly surrendered. The reason: Its commander, whose brother had been executed by Qaddafi years ago, was secretly loyal to the rebellion, a senior rebel official Fathi al-Baja told The Associated Press.

Al-Baja, the head of the rebels' political committee, said the opposition's National Transitional Council had been working on the offensive for the past three months, coordinating with NATO and rebels within Tripoli. Sleeper cells were set up in the capital, armed by rebel smugglers. On Thursday and Friday, NATO intensified strikes inside the capital, and on Saturday, the sleeper cells began to rise up, The Associated Press reported.

BBC world service says Qaddafi is currently in Algeria. Fox News could not confirm the report.

NATO said in a statement that "the sooner Qaddafi 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."

A senior U.S. official, observing the evolving situation in Tripoli, told Fox News that "the momentum that the opposition has built over the past several weeks seems to be paying off. The battle for Tripoli is clearly underway, and what has often seemed impossible--the fall of Qadhafi--may now be attainable."

An Associated Press reporter with the rebels saw them take over the base of the Khamis Brigade, 16 miles west of the capital, on Sunday. The base has been defending Qaddafi's stronghold of Tripoli. After a brief gunbattle, Qaddafi's forces fled.

Qaddafi's 27-year-old son Khamis commands the 32nd Brigade, also known simply as the Khamis Brigade, one of the best trained and equipped units in the Libyan military.

Inside the base, hundreds of rebels cheered wildly and danced, raising the rebel flag on the front gate of a large, gray wall enclosing the compound. They seized large stores of weapons from the base, driving away with truckloads of whatever arms they could get their hands on. One of the rebels carried off a tube of grenades, while another carted off two mortars.

Ahmed al-Ajdal, 27, a fighter from Tripoli, was loading up a truck with ammunition.

"This is the wealth of the Libyan people that he was using against us," he said, pointing to the haul. "Now we will use it against him and any other dictator who goes against the Libyan people."

"Anti-Qaddafi forces have had momentum on their side for some time," a U.S. senior administration official told Fox News on Sunday. "If Tripoli eventually falls to the rebels, Qaddafi's already limited options would become even more limited. Pressure on him and his shrinking circle of loyalists has to be taking a seriously toll."

A Libyan government officials says at least 376 people have been killed in Tripoli during the overnight attacks and over 1,000 were injured.

But a government spokesman also told reporters Sunday that NATO's airstrikes have gone astray, targeting civilian buildings including schools, hospitals, farms and houses.

Anti-regime protests erupted Sunday in several Tripoli neighborhoods where thousands braved the bullets of snipers perched atop high buildings, residents and opposition fighters said.

Mukhtar Lahab, a rebel commander closing in on Tripoli and a former captain in Qaddafi's army, said relatives inside the capital reported mass protests in four neighborhoods known to be sympathetic to the opposition: Fashlum, Souk al-Jouma, Tajoura and Janzour. He said mosques there were rallying residents with chants of "Allahu Akbar" or "God is great," broadcast on loudspeakers.

Snipers on high buildings were firing on protesters in at least one of the four neighborhoods, said Lahab. Residents contacted in the city by telephone also reported snipers firing on civilians.

Fighters said a 600-strong rebel force that set out from Zawiya has reached the outskirts of the village of Jedaim and was coming under heavy fire from regime forces on the eastern side of the town.

The Associated Press contributed to this report.