It appears after months of fighting Gaddafi and his government are falling.
Video from Sky
A convoy of Libyan rebels has rolled into central Tripoli past celebrating crowds after a day of heavy fighting in and around the capital.
They appeared to meet little resistance and civilians emerged to cheer them, waving flags and firing celebratory shots as they passed in their pick-ups.
Video from the city apparently shows the rebels celebrating on Green Square.
The rebels reportedly captured Col Muammar Gaddafi’s son Saif al-Islam as the leader himself vowed to fight on.
“I have received confidential information stating he has been arrested,” International Criminal Court prosecutor Luis Moreno-Ocampo told AFP news agency.
The court has indicted Saif for torturing and killing civilians.
Another of Col Gaddafi’s sons, Muhammad, was also detained, according to rebel sources.
The BBC’s correspondent in Tripoli, Matthew Price, says the rebels appear to be trying to take control of the hotel where he and other journalists have been staying. Gun battles have raged around the Rixos Hotel all day.
Col Gaddafi is believed to still have thousands of armed followers in the capital but other reports suggest a number of them have been surrendering to the rebels.