Street fighting has raged in Sirte for a second day as troops loyal to Libya's transitional government confront the remnants of ex-leader Col Muammar Gaddafi's forces.Interim authority forces seized control of a key boulevard, isolating a conference centre where Gaddafi loyalists have been holed up.Thousands of civilians remain trapped.Once Sirte falls, Libya's leaders say they will declare liberation, even if Col Gaddafi remains on the run."There is a very vicious battle now in Sirte," said National Transitional Council (NTC) chairman Mustafa Abdel Jalil in the capital Tripoli."Today our fighters are dealing with the snipers that are taking positions and hiding in the city of Sirte."Snipers on rooftopsOn Friday, NTC forces launched what they called a final assault on Sirte, pushing pro-Gaddafi fighters back from their positions and towards the city centre.But on Saturday, their rapid advance slowed down as they fought street by street to take control of the city, Col Gaddafi's birthplace and a symbolic second capital for Libya.Pro-Gaddafi snipers fired from the rooftops of the Ouagadougou conference centre, the university and a complex of flats.However, the NTC side won control of a key boulevard which connects the Ouagadougou centre to central Sirte.NTC fighter Faraj Leshersh told Reuters the Gaddafi loyalists were experts at operating unseen, using trenches or burning tyres to give themselves cover to move between buildings."They took advantage of the dust and they advanced a little. There is 500m (yards) between us and them," he said.Civilians continued to leave Sirte, on foot and by car. They were stopped and searched by NTC forces at checkpoints.The NTC gave civilians the opportunity to leave before the assault began.However, thousands remained in the city, unable to get out or fearful after warnings from pro-Gaddafi fighters that they would be attacked by the interim forces if they surrendered.Efforts to negotiate with loyalist commanders have also failed. On Thursday, Col Gaddafi delivered an audio message urging Libyans to take to the streets "in their millions" to resist the interim leaders.Pro-Gaddafi forces also control the desert enclave of Bani Walid, but it is seen as less significant as it does not lead to any exit routes from the country.