Get the latest developments via our LIVE BLOG.
Tripoli, Libya (CNN) -- An amateur league of ill-trained rebel fighters appears to be on the brink of toppling Moammar Gadhafi's 42-year rule after reportedly capturing two of the leader's sons and infiltrating the Libyan capital.
But in a possible indication that the fight is not over, celebrations in Tripoli's Green Square -- renamed Martyrs' Square by the rebels -- gave way to tension Monday morning after rebels told CNN that they'd heard Gadhafi army forces were heading their way. CNN could not confirm any movement of Gadhafi forces.
The uncertainty came hours after a rebel official said two of Moammar Gadhafi's sons -- Saif al-Islam and Saadi -- had been arrested by opposition forces. Jumma Ibrahim, a rebel spokesman based in Libya's western mountain region, said both were captured in Tripoli.
International Criminal Court chief prosecutor Luis Moreno-Ocampo said the court plans Monday to contact authorities associated with those holding Saif al-Islam to try to arrange for his transfer to the Netherlands for an eventual trial for "crimes against humanity." The court, based in The Hague, issued an arrest warrant earlier this summer for Saif Gadhafi as well as his father and his uncle Abdullah al-Sanussi.
Saadi Gadhafi, a businessman and onetime professional soccer player, helped set up an April CNN interview with a woman who claimed she'd been raped by government troops. He later told CNN that those behind the attack should be prosecuted.
There was no immediate reaction from Libyan government officials to the reports of the sons' arrests.
In an audio address broadcast just before midnight Sunday, Moammar Gadhafi claimed "very small groups of people who are collaborators with the imperialists" were fighting inside the capital. Should the rebels prevail, Gadhafi said NATO would not protect them and predicted massive bloodshed. To prevent such bloodshed, he said, Libyans, including women, should go out and fight.
"Get out and lead, lead, lead the people to paradise," he said.
Just after midnight Sunday, scores of raucous rebel supporters packed Green Square -- the same place where Gadhafi loyalists have congregated regularly -- celebrating, waving the rebel flag and even flashing the "victory" sign.
NATO Secretary General Anders Fogh Rasmussen said Sunday that "the Gadhafi regime is clearly crumbling," and urged the leader to acknowledge defeat.
"The rebel fighters are in control of most of the neighborhoods in Tripoli," said Ibrahim, the rebel spokesman.
A main supply route into western Tripoli that had been the scene of intense fighting was clear early Monday, occupied only by rebels heading toward the capital.
CNN's Sara Sidner reported around 3 a.m. that the route heading to Green Square was "eerily quiet," with cars passing by checkpoints run by opposition loyalists. Between 100 and 150 rebel fighters by then had gathered in the square, only to scatter an hour later amid concerns about possible snipers and an upcoming battle there, in the heart of the city.
The advance included members of the "Tripoli Brigade," a group of rebel troops who'd once lived in the capital and could help navigate the city. But they weren't all professional soldiers, such as one IT worker who hadn't held a gun before joining the movement a few months ago.
They entered a city that, after being largely free of large-scale fighting since the conflict began six months ago, became the site of intense drama and significant violence over the weekend.
Libyan government spokesman Musa Ibrahim told reporters just after 11 p.m. Sunday that about 1,300 people had been killed and about 5,000 wounded in fighting in the previous 12 hours.
"(The city) is being turned into a hellfire," he said.
The spokesman denied a report from Arab-language news network Al-Arabiya that Gadhafi's guard had surrendered, calling it "false information."
In another sign of possible trouble for the regime, the signal for state-run television -- which has long been a forum for pro-Gadhafi views -- repeatedly froze, with the station later claiming there had been "interference" due to a "hostile media campaign." The network reverted to taped broadcasts of previous pro-Gadhafi gatherings.
A fierce gun battle broke out Sunday evening near the hotel where many international reporters were stationed in Tripoli. Many government officials packed their suitcases and left the hotel earlier in the day.
A woman in Tripoli said late Sunday that she and others went outside, "screaming" and calling for Gadhafi's ouster -- and had plenty of company.
"We realized that no one wants him, no one wants this dictator," said the resident, whom CNN is not naming for safety reasons.
Musa Ibrahim told CNN on Sunday that "more than 65,000 professional men" are fighting in Tripoli, with thousands more flooding in to help defend the regime, and added they "can hold for much longer." He predicted a "humanitarian disaster" unless an immediate ceasefire is called.
"It's not about who will win," he said. "The world needs to hear this message, that a massacre will be committed in Tripoli if one side wins now."
Some areas of eastern Tripoli, including the suburb of Tajoura, were out of government control Sunday, according to a Libyan government official who asked not to be named. Rebels set car tires afire along barricades there, the official said.
Meanwhile, Zawiya -- a key coastal city about 30 miles west of the capital -- appeared under rebel control, with celebratory gunfire and fireworks as some yelled out, "Libya is free!"
Aref Ali Nayed, an ambassador in the United Arab Emirates for the Libyan rebels' Transitional National Council, said that opposition forces were calling Sunday "Day 1."
"The reason we declare it 'Day 1' is because we feel Gadhafi is already finished. He is already finished, most importantly, in our hearts," he said. "We no longer fear him."
Ibrahim, the government spokesman, blamed NATO for the conflict and appealed for a cease-fire.
"Every drop of Libyan blood shed by these rebels is the responsibility of the Western world, especially NATO's countries," he said. "We hold (U.S. President Barack) Obama, (British Prime Minister David) Cameron and (French President Nicolas) Sarkozy morally responsible for every single unnecessary death that takes place in this country."
Several U.S. officials -- including President Barack Obama, Defense Secretary Leon Panetta and Secretary of State Hillary Clinton -- were getting updates on the situation, officials said.
"Tonight, the momentum against the Gadhafi regime has reached a tipping point," Obama said in a statement, claiming "Tripoli is slipping from the grasp of a tyrant. ... The surest way for the bloodshed to end is simple: Moammar Gadhafi and his regime need to recognize that their rule has come to an end."
In the rebel hub of Benghazi, meanwhile, CNN iReporter Sammi Addahoumi showed video of large, boisterous crowds in the city's Freedom Square reacting as reports of the developments played on a large screen.
"The spirits are quite high," said Addahoumi, a 28-year-old deli manager from South Carolina who said his father fled Benghazi decades ago. "Everyone is expecting Tripoli to fall."
In the first of his speeches on state television Sunday, though, Gadhafi was still insisting the rebels -- whom he described as "infidels," "traitors" and "gangsters" -- would fail and vowed not to back down.
"This is the hour of victory," he said. "This hour is the hour of defiance."
CNN's Sara Sidner, Raja Razek, Jomana Karadsheh, Matthew Chance, Christine Theodorou, Kamal Ghattas, Greg Botelho, Mark Phillips, Kareem Khadder, Roba Alhenawi and Barbara Starr and journalist Mike Mount contributed to this report.