The most recent barrage came after Gaddafi forces brought their tanks to the western gates of Misrata, said Libyan activist Rida al-Montasser. The shelling started up early Monday morning and only paused with the threat of NATO airstrikes, he said.
"Only when we heard the NATO planes flying over, the shelling paused," said al-Montasser.
Foreign nations have been enforcing a no-fly zone over Libya for about a month and a half, but NATO officials now leading the effort have said that pilots are having trouble hitting Gaddafi forces in crowded urban areas like Misrata, which has a population of about 300,000.
Even as the shelling paused Sunday, fear spread through Misrata that Gaddafi forces were preparing to use chemical weapons in their fight to defeat the rebels, who control eastern Libya and have demanded the Libyan leader step down.
"We heard like everybody else that the soldiers are distributing gas masks" in the nearby city of Zlitan, said al-Montasser.
The rumors could not be independently confirmed.
Rebels have asked officials in Benghazi, the de facto rebel capital in eastern Libya, to send a shipment of gas masks to Misrata, said al-Montasser.
Gaddafi pledged to give up his chemical-weapons in 2003 as part of his reconciliation with the West, but some officials have expressed fear that he may still have stockpiles he could use.
On Sunday, the Libyan government unleashed two volleys of rockets on Misrata's port, and heavy shelling occurred elsewhere in the city throughout the day. The attacks killed 12 people, raising the two-day death toll to 23.
Also Sunday, vandals burned Western embassies and a U.N. office in the Libyan capital of Tripoli, hours after NATO bombed Gaddafi's family compound in an attack officials said killed the leader's second youngest son and three grandchildren.
The embassies vandalized Sunday were empty and nobody was reported injured, but the attacks heightened tensions between the Libyan regime and Western powers, prompting the United Nations to pull its international staff out of the capital.
NATO officials and allied leaders emphatically denied the airstrike on Gaddafi's family compound meant they were hunting the Libyan leader to break the stalemate between the better trained government forces and the lightly armed rebels.