Libyan rebels have reportedly entered Tripoli and are within two miles of the city's center, as a report from Sky News says one of Muammar al-Qaddafi's sons has been captured by rebel fighters.

Reports from the country indicate rebels met little resistance Sunday as they overran a major military base that defends the capital.

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.

Sky News reported that Qaddafi's son, Saif al Islam Qaddafi, was captured by rebel forces. He was reportedly the head of the Libyan defense.

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.

Inside Tripoli, there was a second day of widespread clashes between what the opposition called "sleeping cells" of rebels who are rising up and Qaddafi loyalists.

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

Libyan rebels captured a major military base in their assault on Tripoli, giving them access to large stores of weapons.

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."

Libya's government is calling for an immediate cease-fire between rebels and forces loyal to leader Muammar al-Qaddafi after an opposition-led attack, coordinated with NATO airstrikes, rocked the country's capital.

Libyan rebels said they were less than 20 miles from Muammar al-Qaddafi's main stronghold of Tripoli on Sunday, a day after opposition fighters launched their first attack on the city.

NATO has reportedly been bombing Qaddafi's Tripoli compound continuously, though no casualties have been reported yet.

The rebels said Saturday that gunbattles and mortar rounds rocked the city during the attack. NATO aircraft also made heavy bombing runs after nightfall, with loud explosions booming across the city.

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.

NATO said the shifting battle lines and concentration of fighting in towns and villages are making it more difficult to identify and engage targets for the strikes.

"It's much tougher to do in an urban area," NATO spokesman Col. Roland Lavoie said. "This requires very precise and deep intelligence to achieve without endangering the civilian population."

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.

Murad Dabdoub, a fighter who returned to Zawiya from the front, told The Associated Press that Qaddafi's forces were pounding rebel positions with rockets, mortars and anti-aircraft fire.

The rebels' arrival at Jedaim was also confirmed by Abdul-Bari Gilan, a doctor in Zawiya. He told the AP that he had treated a rebel who was wounded in the fighting at Jedaim.

An AP reporter in Tripoli, meanwhile, said the city was largely quiet on Sunday after a night of gunfire and explosions.

Qaddafi's regime was defiant.

"There are thousands and thousands of soldiers who are willing to defend the city," government spokesman Moussa Ibrahim said at a news conference in Tripoli.

Ibrahim, who earlier played down claims of an uprising in Tripoli, accused the rebels of executing innocent civilians, torching homes and robberies. "They are nothing without NATO," he said.

State Libyan television on Saturday aired what appeared to be a live audio message by Qaddafi in which he condemned the rebels as traitors and "vermin" who were tearing Libya apart and said they were being chased from city to city -- a mirror image of reality.

"Libyans wanted to enjoy a peaceful Ramadan," he said. "Instead they have been made into refugees. What are we? Palestinians?" He called on Libyans to march by the millions on cities across the nation to peacefully liberate them.

The claims from both sides could not immediately be independently verified.

Tripoli has been Qaddafi's stronghold since the Libyan civil war began some six months ago, but it is not clear whether the embattled leader was still there.

The capture of Tripoli would almost certainly herald the end of Qaddafi's regime, more than 40 years after the maverick leader seized power in a military coup that toppled the monarchy of the vast North African nation.

Meanwhile, Qaddafi's son and one-time heir apparent Seif al-Islam delivered a similarly defiant message on Saturday when he addressed supporters. He told them: "We are not surrendering; it is impossible to raise the white flag."

"Surrender or the white flag are rejected because this is not the decision of Muammar Qaddafi or Seif al-Islam, it is the decision of the Libyan people," he said.

The Associated Press contributed to this report.