Libyan government forces have launched an attack on the town of Misrata, seeking to retake the last remaining rebel holdout in the west of the country.
"We are hearing shelling. We have no choice but to fight," Gemal, a rebel spokesman, told the Reuters news agency.
Another rebel, Mohamad Ahmed, said he could hear the sound of anti-aircraft guns getting closer to the city centre.
A resident, who gave his name only as Mohammed, confirmed an attack by Gaddafi's loyalists was under way: "I can hear loud explosions, everybody is rushing home, the shops have closed and the rebels are taking positions in the neighbourhoods".
Some government troops fighting for control of Misrata have defected and joined the rebellion, according to a rebel spokesman.
Rebels in Misrata said forces loyal to Gaddafi have failed to retake the rebel-held town.