An AFP correspondent said the fighting force comprised dozens of pick-up trucks fitted with heavy machine guns and flanked by three tanks.
The assault came after National Transitional Council (NTC) forces suffered heavy losses when they launched a first attack on the fugitive Gaddafi''s coastal bastion before sunset yesterday, with 11 fighters killed and 34 wounded.
Gaddafi''s forces appeared today to have retreated to the southern outskirts of the city, the correspondent said, while some fighters reported a battle under way for the airport.
"The rebels have half of Sirte and with God''s will, will take the other half today," said Ibrahim Sweyib of the Red katiba (brigade).
Another fighter, Inneyo, also said half of Sirte is under the control of NTC forces.
"We have residential zone two and now we must take residential zones one and three," Inneyo said.
"They are firing Grad rockets at us," said Abdul Basit Jari, a fighter from the Misrata Soldiers Brigade.
"That black smoke is a Gaddafi loyalist''s car we struck," Jari said, pointing to smoke billowing across the skyline.
The fighters had earlier regrouped at Hassan, some 60 kilometres west of Sirte, to prepare for the assault.
There were dozens of pick-ups bristling with anti-aircraft guns and some with cannon, along with many trucks, some laden with ammunition.
A convoy of battle-hardened fighters had set out from the port city of Misrata, some 150 kilometres west of Sirte, early yesterday before splitting at the crossroads town of Abu Qurin for a three-pronged attack on Sirte.
The military council said the convoy of more than 900 armoured cars aimed at "freeing the city and raising the banner of independence."