Libyan leader Moammar Gadhafi, center, with Egyptian President Hosni Mubarak, right, and his Yemeni counterpart Ali Abdullah Saleh, left, pose during a 2010 group picture with Arab and African leaders during the second Afro-Arab summit in Sirte, Libya. As rebels swarmed into Tripoli, Libya, and Gadhafi's son and one-time heir apparent Seif al-Islam was arrested, Gadhafi's rule was all but over, Western officials say.

By KARIN LAUB and BEN HUBBARD

Associated Press

TRIPOLI, Libya - Libyan leader Moammar Gadhafi was nowhere to be found Monday as his 42-year rule teetered on the brink of collapse.

Months of NATO airstrikes have left his Bab al-Aziziya compound in Tripoli largely demolished. Most of his security forces fled or surrendered when rebel forces rolled into the capital Sunday night and took control of most of the city. Three of his sons were under arrest.

A mood of joy mixed with trepidation settled over the capital, with the rebels still fighting pockets of fierce resistance from regime loyalists firing mortars and anti-aircraft guns. Rebel spokesman Mohammed Abdel-Rahman, who was in Tripoli, said the "danger is still there" as long as Gadhafi remains on the run.

"The real moment of victory is when Gadhafi is captured," Mustafa Abdel-Jalil, head of the rebel National Transitional Council, told a news conference in the opposition's de facto capital of Benghazi, hundreds of miles east of Tripoli. He said the rebels have no idea where Gadhafi is and whether he is even in Tripoli. An Obama administration official said the U.S. had no indication that Gadhafi had left Libya.

Gadhafi's forces remained active, firing off a short-range Scud missile Monday near Sirte, Gadhafi's hometown and one of the few remaining cities still under his control, said U.S. military officials, who spoke on condition of anonymity to discuss military operations. It was unclear where the missile landed or if anyone was hurt.

President Barack Obama said the situation in Libya reached a tipping point in recent days after a five month NATO-led bombing campaign. However, he acknowledged that the situation remained fluid and that elements of the regime remained a threat.

The Obama administration official said U.S. officials and NATO partners had not been in contact with Gadhafi during the siege on Tripoli. However, the official said American and NATO representatives, as well as Libyan rebels, had all been in contact with people around Gadhafi, mostly those looking for a way out.

NATO vowed to keep up its air campaign until all pro-Gadhafi forces surrender or return to their barracks. The alliance's 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 in March, NATO said.

A day after the rebels rode into the city of 2 million, the situation remained volatile. Even though rebels claimed they were in control of most of Tripoli, they still appeared to be on the defensive, ducking for cover during frequent clashes with regime fighters. Throughout the day, the rebels sent reinforcements to the city from the north, south and southeast, and a rebel field commander said more than 4,000 fighters were part of the final push to bring down the regime.

The Obama administration official said the U.S. believes 90 percent of the capital is under rebel control, while regime loyalists still control Sirte and the southern city of Sabha. The official spoke on the condition of anonymity because they were not authorized to speak publically.

The Scud missile fired near Sirte was launched at about 2:15 p.m. EDT (1915 GMT), U.S. officials said.

It was only the second Scud missile fired during this year's conflict. On Aug. 15, Libyan government forces launched one near Sirte that landed in the desert outside Brega, injuring no one.

Intense gun battles erupted throughout the day and the capital was too unstable for any mass celebrations in the streets.

Clashes broke out early in the day at Gadhafi's Bab al-Aziziya compound when government tanks emerged from the complex and opened fire at rebels trying to get in, according to the rebel spokesman Abdel-Rahman and a neighbor.

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 who have not fled or surrendered.

"When I climb the stairs and look from the roof, I see nothing at Bab al-Aziziyah. It is totally deserted except for the house which was raided by U.S. in 1986. Nothing else is there. Gadhafi can't be there," he said. "NATO has demolished it all and nothing remained."

But Abdel-Rahman said Gadhafi still has forces to be reckoned with.

"We know that until now, Tripoli is encircled by Gadhafi brigades positioned at the outskirts of the capital, in camps, such as al-Yarmouk in the south of Tripoli. They can be in the middle of the city in half an hour."

Still, revelers flocked to Green Square, the symbolic heart of the fading Gadhafi regime. They flashed the "V" for victory sign and motorists circled the plaza, honking horns and waving rebel flags.

"We came out today to feel a bit of freedom," Ashraf Halati, 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."

Late Sunday night, rebels took over Green Square, which they have been calling Martyrs' Square, restoring the name it had before Gadhafi's regime took power more than four decades ago. Google's map of Tripoli has already adopted the new name. The opposition also took up the pre-Gadhafi flag of Libya as their own at the start of their uprising six months ago.

On Monday, rebels manned checkpoints on the western approaches to the city, handing out candy to motorists and inquiring about their destinations.

Most of the city, a metropolis of some 2 million people on the Mediterranean coast, was on edge. Stores were shuttered and large areas were lifeless, including the old gold market, in the past a draw for tourists.

Around midday Monday, rebel fighters took over a women's police college near the Mediterranean and declared that they would set up their new headquarters there.

"We are going to protect the city of Tripoli from all attacks and threats," fighter Munir al-Ayan said after kneeling and kissing the ground in the compound.

"I was bowing down to the Almighty God who helped us get rid of this brutal dictator," he later explained.

The compound's previous inhabitants appeared to have left in haste, and their belongings were not touched.

In one office, women's green police uniforms dangled from hangers, and black dress boots stood nearby. A computer and a TV screen were still in place. Pictures of Gadhafi adorned the wall. In another room, the police marching band's instruments were stacked next to a wall, including drums and a trumpet.

But the rebels' optimistic mood of the morning quickly changed. By mid-afternoon, the college came under heavy fire. Snipers from nearby high-rises aimed at motorists speeding by. An anti-aircraft gun pounded the compound, creating a deafening noise. A handful of rebel fighters inside seemed jumpy and unsure what to do.

Gadhafi loyalists also launched attacks in two other areas of Tripoli, said Ashraf Hussein, a rebel fighter who sat pressed against an inner wall of the compound for safety.

Drivers trying to evade sniper fire ducked into side streets, or stopped at rebel checkpoints to find out whether the next stretch was safe. Booms of mortar rounds and small rockets reverberated across the city, mixed with battle cries of "Allahu akbar," or God is Great.

Later Monday, another battle erupted around a school where rebels and journalists had set up camp. Rebels fired small rockets, and Gadhafi troops responded with mortar shells.

The fighting made clear that taking full control of Tripoli will be difficult, especially as long as Gadhafi has not surfaced.

There were also signs of tension 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. Opposition leaders urged people to protect public property, and no looting was reported.

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.

About 30 journalists remained in the hotel where armed pro-Gadhafi youths kept a close eye on them and did not allow to them to exit the building. Journalists began to worry that food, water and fuel that powers the hotel's generator were running low.

Some of the journalists attempted to walk out of the hotel but were met with hostility by the armed guards, who said they were put there to "protect" them. Journalists said they felt like they were being held hostage.

Outside of Tripoli, almost all of eastern and western Libya is now under rebel control. The east of the country from the Egyptian border to Benghazi fell into rebel hands at the beginning of the uprising. In the weeks leading up to Sunday's lightning advance on Tripoli, the rebels consolidated control of the western Nafusa mountain range near the border with Tunisia. It was from there they staged the run on the capital. Most of the rest of the country was quickly falling into their hands.

The city of Sirte, Gadhafi's hometown to the east of Tripoli, was the most important loyalist bastion to remain fully under his control.

On Monday, the city was without power and full of heavily guarded Gadhafi checkpoints, said Hassan al-Daroui, an official with the rebel council in Benghazi who was in touch with people there by satellite phone. Many people there were not even aware that rebels had pushed into the capital, 250 miles (400 kilometers) to the northwest, he said.

On Saturday rebels said they gained control of the oil refineries and airport at the oil terminal of Brega, on the road heading out of Benghazi west toward Tripoli.

The rebels' startling breakthrough on Sunday, 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, 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. 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.

"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," Abdel-Jalil said.

Libyan state television was off the air Monday amid reports it had been seized by rebels.

Abdel-Jalil said rebels have now captured three of Gadhafi's sons. He said they detained al-Saadi Gadhafi on Sunday night along with his brother Seif al-Islam, the one-time heir apparent to his father.

Gadhafi had seven biological sons and a daughter who all played roles in their father's regime, some in diplomacy and others in business. Al-Saadi and his brothers Mutassim and Khamis all headed military brigades. One of his sons was killed in the civil war.

The International Criminal Court has confirmed the capture of Seif al-Islam, who along with his father faces charges of crimes against humanity. Another son, Mohammed, was under house arrest.

Abdel-Jalil began to look ahead to the task his council might face in shepherding a transition in power.

"We have much challenges and responsibilities, starting by healing the wounds, " he said, adding the rebels wanted to ensure safety, security, peace and prosperity.

"Gadhafi will be remembered through the acts he committed in Libya and the world, by political assassinations, arrests and executions and oppression of all the Libyan people who wanted to oust him from the beginning of this revolution."

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