"Initial statistics: 11 martyrs and 34 wounded," an English-language statement by the Misrata Military Council said.
It added that it had captured 40 Kadhafi loyalists.
According to the military council, their fighters stormed from three front on Thursday into the Mediterranean city, one of Kadhafi's last pockets of resistance.
A military spokesman, Fathi Bashaga, said a "big force" had entered Sirte.
"There is still resistance but our fighters will be able to overcome it," he told an AFP correspondent.
"They are attacking us with 40- and 43-mm mortars and all kinds of weapons."
A convoy of battle-hardened fighters had set out from the port city of Misrata, about 150 kilometres (93 miles) west of Sirte, early on Thursday 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."