The New York Times reported by the time night fell, the rebels had raced into the center of the capital and had gained control of Green Square, long emblematic of Gadhafi's control of the country.
Exuberant Libyans tore down Gadhafi portraits and the green flags that marked his rule, while buoyant crowds cheered and waved rebel flags in the public grounds now renamed Martyrs' Square.
"My country is free! God is great! My country is free!" one Gargaresh resident told the Times when reached by telephone.
A cautious calm overtook the city by 3 a.m. as rebel fighters secured the entrances to the square, the U.S. newspaper said.
Where Gadhafi was holed up remained a mystery, ABC News reported, with the rebels saying they have captured one of Gadhafi's sons, Saif, in the Rixos hotel in Tripoli.
U.S. President Barack Obama, on vacation at Martha's Vineyard in Massachusetts, said in a statement Sunday night that "the momentum against the Gadhafi regime has reached a tipping point."
"Tripoli is slipping from the grasp of a tyrant," he said. "Gadhafi regime is showing signs of collapsing. The people of Libya are showing that the universal pursuit of dignity and freedom is far stronger than the iron fist of a dictator.
"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. Gadhafi needs to acknowledge the reality that he no longer controls Libya. He needs to relinquish power once and for all."
Obama said the United States has recognized the Transitional National Council as the legitimate governing authority in Libya.
"At this pivotal and historic time, the TNC should continue to demonstrate the leadership that is necessary to steer the country through a transition by respecting the rights of the people of Libya, avoiding civilian casualties, protecting the institutions of the Libyan state, and pursuing a transition to democracy that is just and inclusive for all of the people of Libya," Obama said. "A season of conflict must lead to one of peace."
If Gadhafi was still in Tripoli, rebel forces believed he was in his primary compound, called Bab al-Aziziya, Mohamad al-Akari, a Transitional-National-Council adviser, said. He added, "Tonight it's over."
ABC said the rebels overran a major military base that defended Gadhafi's stronghold.
Earlier, senior NATO and British officials said the air war in Libya may wind down as rebels engage in street-by-street fighting in Tripoli.
Though NATO was looking toward the end of operations, unnamed officials had told Britain's The Guardian newspaper they had been concerned the fight in the streets of Tripoli might be extremely bloody.
"What NATO can do [in support of the rebels] is very limited if there is street-by-street fighting," one British defense official-told the newspaper, while another added, "In house-to-house fighting [NATO] will not be able to use air power."
In the run-up to the rebel invasion of Tripoli, Royal-Air Force Tornado jets had hit the Libyan Ministry of Defense, The Guardian said, a key communications facility in southwestern part of Tripoli. Tornado and Typhoon bombers also attacked command and control facilities in Tripoli Saturday, the newspaper said.
Libya's besieged government had futilely called for a cease-fire Sunday.
A government spokesman issued a statement demanding a halt to the offensive, saying Tripoli was well defended and that the rebels were no match for Gadhafi's army without significant backing from NATO, Britain's Sky News reported.
"We have thousands upon thousands of professional-soldiers ready to defend this city against any possible invasion by rebels under the cover of NATO," the statement said before the city fell to the rebels.
Media reports Sunday said running gun battles in Tripoli had ebbed and flowed and Gadhafi's forces remained in control of the waterfront and the eastern reaches of city.
"I think (Gadhafi) has lost most of the city," one dissident leader told The Washington Post by telephone during a lull in the fighting. "We have information that they are planning an attack on us soon."
Another rebel leader, Mustafa Abdul Jalil, told the Los Angeles Times: "The end is very near. I expect a catastrophic end for him and his inner circle."