Libyan leader Moammar Gaddafi’s four-decade-long rule over the country was crumbling at breakneck speed as hundreds of rebel fighters swept into Tripoli and took control Monday of the symbolically significant Green Square in the heart of the city.

With rebel leaders saying late Sunday that Gad­dafi’s compound was surrounded, that his son Saif al-Islam had been captured and that his presidential guard had surrendered, the six-month-old battle for control of Libya appeared to be hurtling toward a dramatic finale.

In a written statement, President Obama said: “Tonight, the momentum against the Qadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant.”

Early Monday there were reports of heavy clashes near Gaddafi’s compound. Rebel spokesman Mohammed Abdel-Rahman said that tanks had emerged from the complex and began firing, AP said.

Only a few residents in Tripoli ventured out to greet the rebels, and there was an overall sense of nervousness on Green Square. Posters of Gaddafi had been ripped to pieces, and young men were shooting off guns and fireworks.

In a brief broadcast on state television, Gaddafi made what came across as a desperate plea for support. “Go out and take your weapons,” the Libyan leader said. “All of you, there should be no fear.”

The rebel advance had unfolded with surprising speed throughout the day as fighters converged on the capital from three directions. In areas under rebel control, opposition flags fluttered, while jubilant residents honked horns, set off fireworks and stomped on posters of Gaddafi.

With communications to the capital sporadic, some rebel claims could not be confirmed, and some experts cautioned that a tough urban battle may yet lie ahead between the lightly armed and untrained rebels and the elite government forces kept in reserve for the defense of the capital.

But reporters traveling with rebel forces said Gaddafi’s defenses were melting away faster than had been expected. There were reports of entire units fleeing as rebels entered the capital from the south, east and west, and his supporters inside the city tearing off their uniforms, throwing down their weapons and attempting to blend into the population.

A Tripoli-based activist said the rebels had secured the seaport, where several hundred reinforcements for the opposition had arrived by boat, and were in the process of evicting Gaddafi loyalists from the Mitiga air base on the eastern edge of the city.

“The Gaddafi regime is clearly crumbling,” said a statement issued by NATO, whose five-month-old aerial bombing campaign, ostensibly launched to protect civilians from attacks by the government, contributed enormously to the erosion of Gaddafi’s defenses.

NATO said Monday it will continue its combat patrols over Libya until Gaddafi loyalists surrender or return to barracks.

A U.S. official closely tracking intelligence reports from Tripoli said there was no independent confirmation of reports that Gaddafi’s sons had been captured but added that events were moving so rapidly that confirmation was difficult. The official said rebel forces appeared to be benefiting not only from strong momentum but also from smart strategic planning going into the capital.

“We could be watching the game-changer unfold,” said the official, who spoke on the condition of anonymity to discuss intelligence reports. “Whether or not Gaddafi reads the tea leaves the same way is the big question.”

In the rebel capital, Benghazi, where huge crowds gathered to celebrate what they hoped was the imminent capture of Tripoli, Transitional National Council leader Mustafa Abdel Jalil announced that Gaddafi’s son Saif al-Islam had been captured.

Rebels in Tripoli also said that Gaddafi’s eldest son, Mohammed, had surrendered after fighters stormed his home.

There was no information about Gaddafi’s whereabouts, though he had issued a defiant speech earlier in the day in which he insisted that he was in Tripoli and would not surrender. “We cannot go back until the last drop of our blood. We will defend the city. I am here with you,” he said in the audio statement, purportedly broadcast live. “Go on, go forward!”

But it appeared that his control had already unraveled as the rebels swept into the capital, encountering only pockets of resistance along the way.

The rebels urged journalists not to remain on the square for too long, illustrating the general sense of uneasiness about their victory. There were rumors that Gaddafi’s forces would try to retake the square, but that did not happen.

“This is the happiest day of my life,” said Abdul Hamid, who had driven in from outside Tripoli to be on Green Square.

People on the square said that rebels had seized the symbolic location around 10 p.m. and that they faced little resistance.

Checkpoints and neighborhood watches had been set up all over the western part of the city, but on the road from the rebel-held city of Dawiya, hundreds of cars and trucks could be seen bringing reinforcements to secure the capital.

“I never thought I’d see a day like this, it’s like our independence day. This is the end of the colonel,” said Adel Bibas, as he stood on a street in the western portion of Tripoli. Elsewhere, there were rumors that Gaddafi’s forces were on the way, and young men urged people to seek refuge inside, a sign of the tension that still weighs on the capital.

After initially seizing control of the strategic western town of Zawiyah last week, the rebels pushed rapidly east throughout the day, capturing a major military base that was home to the Khamis Brigade, an elite force led by Khamis Gaddafi, one of the Libyan leader’s sons.

Exultant rebels seized weapons from the base and were seen carrying away boxes of brand-new Belgian munitions, as others raced by in trucks filled to the brim with weaponry.

By nightfall, the rebel force
had reached suburban Janzour, where witnesses said government forces had abandoned their posts earlier in the day. Residents took to the streets to cheer the rebels as they swept past in their pickups into the southern edges of the city.

At the same time, rebels advancing along the eastern coastal highway were reported to have linked up with opposition fighters in the strategically located eastern suburb of Tajura, long a stronghold of opposition to Gaddafi, effectively cutting off the capital.

In a late-night briefing for journalists confined to the Rixos hotel, government spokesman Moussa Ibrahim said that at least 1,300 people had been killed since noon, in addition to 930 the previous day, and blamed NATO for the bloodshed. The figures could not be independently confirmed, and though rebels said there had been many deaths, they were skeptical that the number was that high.

The lightly armed opponents, who have spent months quietly organizing for this moment, were within a mile and a half of Gaddafi’s Bab al-Aziziya compound by Sunday night. They were hoping to launch an assault on the headquarters as soon as they linked up with the rebel reinforcements arriving in the city, according to Tripoli resident and rebel organizer Abdel Azzuz.

He said that the rebels are convinced that Gaddafi is still in Tripoli and that they are in the process of setting up checkpoints across the city in case he tries to slip away.

A rebel leader who asked to be identified only as Haj said the Libyan independence flag, which has been adopted as the symbol of the opposition, was flying from numerous mosques, government buildings and a shopping mall in areas they had seized. He said fighters could be heard singing the rebel national anthem, the words “Oh, my country” floating through the streets amid the near-constant crackle of automatic-weapons fire.

Haj said he was confident that the opposition fighters could hold out at least another day until the rebel army arrives. “With the efforts of our revolutionary youths and our children, we will be able to make it through tonight,” he said.

He seemed unsure whether they would last longer than that, however, and it was unclear what level of control the anti-Gaddafi forces exert in the neighborhoods they claim to have seized. The rebels said they have a good supply of Kalashnikovs, smuggled into the city covertly since the initial uprising was crushed in March, as well as 9mm pistols and homemade bombs. In Tajura, the opposition forces had seized a sizable quantity of weapons from a government unit that fled, according to Azzuz, the rebel organizer.

In Washington, a second U.S. official said: “The opposition is gaining ground and putting more pressure on the regime each day. When this translates into a tipping point and what the endgame will look like is hard to determine. Gaddafi isn’t sure what he’s going to do from one moment to the next.”

State Department spokes­woman Victoria Nuland said officials there are turning their attention to a post-Gaddafi Libya. “Gad­dafi’s days are clearly numbered,” she said. “If Gaddafi cared about the Libyan people, he would step down now.”

Sly reported from Beirut. Staff writers Joby Warrick in Washington and Leila Fadel in Mersa Matruh, Egypt, contributed to this report.