Forces loyal to Libya's transitional government have launched a major assault on the city of Sirte, one of the last Gaddafi strongholds.Hundreds of vehicles have advanced on the city from both the east and the west and are close to the centre.Thousands of civilians have already left Sirte but many more are thought to have remained behind.The BBC's Jonathan Head, on the city's outskirts, says it is by far the biggest assault in recent days.Sustained tank and mortar fire has been targeting Sirte and there are huge columns of smoke across the city, some 360km (225 miles) east of Tripoli, with many buildings struck and on fire, says our correspondent.This appears to be the final push for Col Gaddafi's home town, he adds, as the country has been unable to think about the future until Sirte falls.The interim authority's troops are coming from both Misrata in the west and Benghazi in the east.The Benghazi forces are only 1km from the city centre but have faced heavy resistance from snipers inside the city. Continue reading the main storyLibya forces in big Sirte assaultThe Ouagadougou conference centre, where many of the Gaddafi loyalists are believed to have gathered, has become the focus of the artillery barrages.On Thursday, an NTC military commander Col Abdel Salam Gadallah said three quarters of the city was in their hands."God willing, in two days maximum all of Sirte will be clean."The NTC troops have given civilians in Sirte the opportunity to leave, but there are fears that thousands have been unable to do so or believed warnings by pro-Gaddafi fighters that they would be attacked by the interim forces if they surrendered.Efforts to negotiate with loyalist commanders have also failed.The attack on Sirte came just hours after Col Gaddafi urged Libyans to take to the streets "in their millions" to resist the interim leaders.In a poor quality audio message broadcast on Thursday on Syrian-based Arrai television, he said conditions in Libya had become "unbearable" and that the National Transitional Council (NTC) were not the country's legitimate leaders."I say to them, do not fear anyone. You are the people, you belong to this land," said Col Gaddafi, telling people to make their voices heard against the "Nato's collaborators" of the NTC.The ousted leader's whereabouts remain unknown. Several of his family member are in hiding or have fled the country.