And Muammar Gaddafi’s son Mo-Tassim has also been killed, NTC information minister was quoted as saying by Reuters.
The minister said he was hiding with his father.
National Transitional Council (NTC) Prime Minister Mahmoud Jibril told a news conference in Tripoli it was time to launch a new, united Libya.
Col Gaddafi was toppled in August after 42 years in power.
The NTC now plans to officially announce Libya’s “liberation” before indicating the next steps towards democratic elections.
Mr Jibril said NTC forces were now pursuing Saif al-Islam, Col Gaddafi’s most prominent son, who fled Sirte in a convoy before Sirte fell.
After a day of conflicting reports and rumours, Mr Jibril told the news conference: “We have been waiting for this moment for a long time. Muammar Gaddafi has been killed.”
World leaders welcomed the news, urging the NTC to carry through its promise to reform the country.
UK Prime Minister David Cameron, who had taken a leading role in the Nato intervention, said it was “a day to remember all of Col Gaddafi’s victims”.
UN Secretary General Ban Ki-moon called it a “historic” moment, but warned: “The road ahead for Libya and its people will be difficult and full of challenges.”
Earlier, grainy video footage circulated among NTC fighters appearing to show Col Gaddafi’s corpse.
The video shows a large number of NTC fighters yelling in chaotic scenes around a khaki-clad body, which has blood oozing from the face and neck.
Another video broadcast by al-Jazeera TV showed a body being dragged through the streets which the channel said was that of Col Gaddafi.
An NTC fighter told the BBC he found Col Gaddafi hiding in a hole in Sirte, and the former leader begged him not to shoot.
The fighter showed reporters a golden pistol he said he had taken from Col Gaddafi.
Arabic TV channels showed images of troops surrounding two large drainage pipes where the reporters said Col Gaddafi was found.