On the third day of what was touted as a final assault, National Transitional Council (NTC) fighters captured the Mediterranean city's showpiece conference centre, university campus and hospital, AFP correspondents said.
The gains are seen as crucial by the NTC, which awaits Sirte's full capture to declare the liberation of the whole of Libya, clearing the way to draw up a timetable for elections.
The council has ruled most of the oil-rich country since its forces overran Tripoli on August 23, forcing Gaddafi and his inner circle to flee.
On another front where Gaddafi holdouts have put up stiff resistance, taking NTC leaders by surprise, their fighters took control on Sunday of the airport of Bani Walid, an oasis 170 kilometres southeast of Tripoli.
NTC fighters have been stationed for weeks outside Bani Walid.
Heavy fighting raged yesterday around a kilometre from the downtown area, which residents have abandoned, according to the military joint command in Tripoli.
The NTC forces have been trying for a month to gain control of Bani Walid.
In Sirte, the fortress-like Ouagadougou conference centre, constructed to host pan-African summits, had been a major objective of NTC forces since they launched a September 15 offensive on Sirte.
