TRIPOLI, Libya (AP) -- Libyan rebels claimed to be in control of most of the Libyan capital on Monday after their lightning advance on Tripoli heralded the fall of Moammar Gadhafi's nearly 42-year regime. Scattered battles erupted, and the mercurial leader's whereabouts remained unknown.

The international community called on Gadhafi to step down and moved ahead with post-war planning as euphoric residents celebrated in the Green Square, the symbolic heart of the Gadhafi regime. Colleagues warned he wouldn't go easily. Two of his sons were captured late Sunday.

"The real moment of victory is when Gadhafi is captured," the head of the opposition's National Transitional Council, Mustafa Abdel-Jalil, said at a news conference in the eastern city of Benghazi.

NATO promised to maintain its air campaign until all pro-Gadhafi forces surrender or return to barracks. NATO warplanes have hit at least 40 targets in and around Tripoli in the past two days - the highest number on a single geographic location since the bombing started more than five months ago, the alliance said.

"We came out today to feel a bit of freedom," Ashraf Halaby, a 30-year-old Tripoli resident, said as he and four of his friends watched several hundred people celebrating at Green Square. "We still don't believe that this is happening."

Revelers flashed the "V" for victory sign and motorists circled the square's central median honking their horns and waving rebel flags.

The rest of the city, a metropolis of some 2 million people on the Mediterranean coast, was on edge, with most stores shuttered and large areas appeared lifeless, without even a sign of the thousands of rebels now in the city.

Signs of tension emerged between rebels and residents at a gas station in the neighborhood of Gourji, with heated arguments over who should fill up first after rebels cut in line. Rebel leaders urged people to protect public property, and no looting was reported.

The rapid rebel advance into Tripoli in an hours-long blitz showcased the evolution of the opposition fighters who first rose against the regime six months ago, swiftly capturing the eastern part of the vast, oil-rich North African nation but failing to advance westward toward Tripoli even with the help of months of NATO airstrikes.

For months, the rebels - mainly civilian volunteers who took up arms and had little military training - were judged to be big on zeal but short on organization and discipline, but their stunning success in Tripoli showed a high level of planning, coordination and discipline.

The U.S. and other nations have recognized the National Transitional Council as Libya's legitimate government, but the rebel movement consists of Islamists as well as former government insiders and Western-leaning intellectuals, raising concern about whether the factions can unite in a post-Gadhafi Libya.

Abdel-Jalil sought to allay those worries at a news conference in the rebel capital of Benghazi, saying the opposition wanted a nation built on the principles of "freedom, equality and transparency."

In London, British Prime Minister David Cameron said frozen Libyan assets would soon be released to help the country's rebels establish order, saying Gadhafi's regime was "falling apart and in full retreat."

Rebel spokesman Mohammed Abdel-Rahman, who was in Tripoli, warned of pockets of resistance and said as long as Gadhafi remains on the run the "danger is still there."

Clashes broke out early Monday at Gadhafi's longtime command center known as Bab al-Aziziya early Monday when government tanks emerged from the complex and opened fire at rebels trying to get in, according to Abdel-Rahman and a neighbor. An AP reporter at the nearby Rixos Hotel where foreign journalists stay heard gunfire and loud explosions from the direction of the complex.

Moammar al-Warfali, whose family home is next to the Gadhafi compound, said there appeared to be only a few tanks belonging to the remaining Gadhafi forces that have not fled or surrendered.

"When I climb the stairs and look at it from the roof, I see nothing at Bab al-Aziziya," he said. "NATO has demolished it all and nothing remains."

The Rixos hotel where foreign journalists are staying also remained under the control of Gadhafi forces, with two trucks loaded with anti-aircraft machine guns and pro-regime fighters and snipers posted behind trees. Rebels and Tripoli residents set up checkpoints elsewhere in the city.

Britain's Defense Secretary Liam Fox said resistance was coming mainly from foreign mercenaries, rather than Libyans still loyal to Gadhafi. "There is a certain amount of violence still occurring, we also know that a lot of the resistance from the pro-Gadhafi forces has in fact come from mercenary elements," he told BBC radio.

"It does appear that a lot of the Libyan forces themselves inside Tripoli either stayed at home or put down their arms - and that may bode well for a diminishing level of violence during the transitional period," he said.

The rebels' top diplomat in London, Mahmud Nacua, said opposition forces controlled 95 percent of Tripoli. He vowed "the fighters will turn over every stone to find" Gadhafi and make sure he faced justice.

A rebel field commander said reinforcements were arriving in Tripoli by sea from the north, south and southeast.

"Our fighters are coming from all directions and, God willing, today we will liberate the whole city," the commander, Suleiman Sifaw, told The Associated Press.

State TV broadcast bitter audio pleas by Gadhafi for Libyans to defend his regime as the rebels advanced on Sunday, although the station was off the air by Monday afternoon amid reports that the rebels had seized its main offices.

Opposition fighters captured Gadhafi's son and one-time heir apparent, Seif al-Islam, who along with his father faces charges of crimes against humanity at the International Criminal Court in the Netherlands. Another son, Mohammed, was under house arrest.

Abdel-Jalil, the rebel chief, vowed Monday to give Gadhafi a "fair trial with all legal guarantees" when captured.

But Gadhafi's defiant audio messages raised the possibility of a last-ditch fight over the capital, home to 2 million people. Gadhafi, who was not shown, called on supporters to march in the streets of the capital and "purify it" of "the rats."

Government spokesman Moussa Ibrahim also claimed the regime has "thousands and thousands of fighters" and vowed: "We will fight. We have whole cities on our sides. They are coming en masse to protect Tripoli to join the fight."

Gadhafi's former right-hand man, who defected last week to Italy, said the longtime leader would not go easily.

"I think it's impossible that he'll surrender," Abdel-Salam Jalloud said in an interview broadcast on Italian RAI state radio, adding that "He doesn't have the courage, like Hitler, to kill himself."

Jalloud, who was Gadhafi's closest aide for decades before falling out with the leader in the 1990s, fled Tripoli on Friday, according to rebels.

The startling rebel breakthrough, after a long deadlock in Libya's 6-month-old civil war, was the culmination of a closely coordinated plan by rebels, NATO and anti-Gadhafi residents inside Tripoli, rebel leaders said. Rebel fighters from the west swept over 20 miles (30 kilometers) in a matter of hours Sunday, taking town after town and overwhelming a major military base as residents poured out to cheer them. At the same time, Tripoli residents secretly armed by rebels rose up.

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

On Monday, rebels erected checkpoints on the western approaches to the city, handing out candy to passengers and inquiring about their destination. Cars leaving the city were subjected to more rigorous checks.

President Barack Obama said Libya is "slipping from the grasp of a tyrant" and urged Gadhafi to relinquish power to prevent more bloodshed.

"The future of Libya is now in the hands of the Libyan people," Obama said in a statement from Martha's Vineyard, where he's vacationing. He promised to work closely with rebels.

South Africa, which led failed African Union efforts to mediate between the rebels and Gadhafi, refused to offer support to the rebels on Monday, saying it wants to see a unity government put in place as a transitional authority.

The uprising against Gadhafi broke out in mid-February, inspired by successful revolts in Egypt and Tunisia, Libya's neighbors to the east and west respectively. A brutal regime crackdown quickly transformed the protests into an armed rebellion. Rebels seized Libya's east, setting up an internationally recognized transitional government there, and two pockets in the west, the port city of Misrata and the Nafusa mountain range.

Gadhafi clung to the remaining territory, and for months neither side had been able to break the other.

In early August, however, rebels launched an offensive from the Nafusa Mountains, then fought their way down to the Mediterranean coastal plain, backed by NATO airstrikes, and captured the strategic city of Zawiya.

Gadhafi is the Arab world's longest-ruling, most erratic, most grimly fascinating leader - presiding over this North African desert nation with vast oil reserves and just 6 million people.

For years, he was an international pariah blamed for the 1988 bombing of a Pan Am jumbo jet over Lockerbie, Scotland, that killed 270 people. After years of denial, Gadhafi's Libya acknowledged responsibility, agreed to pay up to $10 million to relatives of each victim, and the Libyan rule declared he would dismantle his weapons of mass destruction program. That eased him back into the international community.

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Associated Press writers David Stringer in London and Slobodan Lekic in Brussels contributed to this report.

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