Rebels entered Tripoli on Sunday from the west, after a coordinated move by rebel cells in Tripoli late Saturday, and with stunning pace advanced toward the center of the city, meeting almost no resistance along the way.
However, tanks, snipers and machine gun-laden trucks loyal to Gaddafi fought back on Monday, firing "randomly in all directions whenever they heard gunfire."
And Gaddafi, 69, urged civilians to take up arms against rebel "rats," saying in an audio broadcast that he was in the city and would be "with you until the end," Reuters reports.
A rebel spokesman, Mohammed Abdel-Rahman, said tanks emerged from Gaddafi's complex, known as Bab al-Aziziya,in the heart of Tripoli and began firing shortly after dawn on Monday.
Nouri Echtiwi, another rebel spokesman in Tripoli, told Reuters: "Four hours of calm followed the street celebrations. Then tanks and pick-up trucks with heavy machine guns mounted on the back came out of Bab al-Aziziya ... and started firing and shelling. They fired randomly in all directions whenever they heard gunfire."
However, there was no firm information on where the country's leader of 42 years was Monday, and little sign of popular opposition to the rebel offensive.
Gaddafi's sons — including his heir apparent, Saif Al-Islam, wanted by an international criminal court — have reportedly already been arrested.
Rebel fighters moved from building to building on Monday, hunting loyalist sharpshooters, Reuters reports.
Many streets in the city center, where anti-government supporters had celebrated hours earlier, were abandoned owing to the dangers posed by pro-Gaddafi fighters, Al Jazeera reports.
Rebels had already been celebrating on the streets of the capital since Sunday, with one rebel, named Abdullah, telling a Reuters reporter over the sound of gunfire and shelling: "Gaddafi is finished. Now we are free." (GlobalPost photos: Libyans Celebrate as Rebels Take Tripoli) "There are still some pockets of support for Gaddafi. Maybe there is some fighting in some area, but on the whole our fighters control 95 percent of the city [Tripoli] and the country," Mahmud Nacua, a spokesman for the rebels' National Transitional Council told reporters in London. Franco Frattini, foreign minister of Italy — the former colonial power in the North African desert state — confirmed that Gaddafi's forces now controlled no more than 10 or 15 percent of the capital, and said that "time has run out" for Gaddafi, Reuters reports.
Governments around the world, meantime, urged Gaddafi to end the bloodshed and surrender.
U.S. President Barack Obama led the calls for Muammar Gaddafi to relinquish power after six months of rebellion and NATO bombings culminated in a lightning rebel advance into the heart of Tripoli over the weekend.
And British Prime Minister David Cameron released a statement Monday saying:
"The latest information is that the vast majority of Tripoli is now controlled by free Libyan fighters, although fighting continues — and some of it is extremely fierce.
"We have no confirmation of Gaddafi’s whereabouts, but at least two of Gaddafi's sons have been detained.
"His regime is falling apart and in full retreat.
"Gaddafi must stop fighting, without conditions — and clearly show that he has given up any claim to control Libya."
Amid warnings from world leaders "of a risk of a longer, anarchic civil war," Libya's opposition rebels said they intended to set up a set up a transitional government by moving the NTC — first set up by rebels in the eastern city of Benghazi — to Tripoli.
Dow Jones newswire quoted Nacua, the NTC spokesman in London, as saying.
"There is a plan...There will be no [power] vacuum. NTC will move soon from Benghazi to Tripoli and they will appoint a new transitional government which will rule the country."
Cameron's statement read that:
"The National Transitional Council have been planning for this for months and we have been helping with that work.
"Diplomatically, we have a strong mission already in Benghazi consisting of Foreign Office, military and aid specialists, and we will establish a British diplomatic presence in Tripoli as soon as it is safe and practical to do so.
"This will include stabilisation experts who have been planning for this moment with the NTC for months.
"At the UN, we will also be taking early action in the Security Council to give the new Libyan authorities the legal, diplomatic, political and financial support they need."
Nacua, meantime, told reporters that he thought Gaddafi was still in Libya, and that rebel fighters would search for him and arrest him and put him in court."
The EU, meantime, has said it will maintain its sanctions against the Libyan government for the time being but can lift them "fairly rapidly" when the time is right, foreign policy chief Catherine Ashton told reporters Monday.