Doctors in the Libyan city of Misrata say at least 31 people were killed and more than 150 wounded in a artillery attack by troops loyal to Muammar Gaddafi.
Misrata is the main rebel stronghold in western Libya and the country's largest port.
The rebels say this is the most sustained attack the city has faced since they took it over.
The uprising began in mid-February. Misrata was beseiged by government forces for 70 days.