"Gaddafi's forces are bombing the city with artillery shells and tanks. We now have 25 people dead at the hospital, including several little girls," Dr Khaled Abou Selha told Reuters by satellite phone.
"They are even bombing ambulances. I saw one little girl with half of her head blown off," he said, crying.
The doctor and another resident, Mohamed, said the city was still being heavily shelled despite a rebel claim that the attack had been defeated and the announcement at around 1230 GMT by the foreign minister of a ceasefire.
"There are 20 tanks in the city, they are killing everybody because they want to recapture the city by this evening," Mohamed said. The sound of heavy artillery could be heard in the background.
A rebel fighter had earlier said the insurgents had beaten back the attack, despite the heavy weapons used by Gaddafi's forces and the fact that the city of 300,000, the last big rebel stronghold in western Libya, has been under siege for days.
In Tripoli, Foreign Minister Moussa Koussa said Libya had decided to halt all military operations in the country to protect civilians and comply with a United Nations resolution passed overnight.
Several residents said there was no sign of a ceasefire on the ground, and that government tanks were closing in on the centre of the city, about 200 km (130 miles) east of Tripoli.
Tariq, a doctor based in Britain who has been regularly calling residents inside the city, said: "They are still shelling as we speak. The foreign minister obviously lives in a different time zone. It's indiscriminate."
Rebels said the attack on Misrata started at 7 a.m. (0500 GMT), hours after the U.N. Security Council passed a resolution endorsing a no-fly zone and military attacks on Gaddafi's forces to protect civilians.
Shells hit several mosques, schools and residential buildings, they said.
The reports could not be independently verified because journalists are prevented from travelling to the city.
"It's the heaviest bombardment I have seen so far. We believe they (Gaddafi's forces) want to enter the city at any cost before the international community starts implementing the U.N. resolution," said Saadoun, a rebel fighter.
"On behalf of all the people of Misrata, the women, the children and the elderly, we call on the international community to do something before it's too late. They must act now," he said. "They already failed us before and were late in taking a decision, they should not repeat the same mistake."
Gaddafi's forces have repeatedly attacked Misrata in the past two weeks. Water supplies have been cut off, there are frequent power cuts and communications are very difficult, residents said.
There were also reports of fighting further west, near the border with Tunisia. Rebels in the town of Nalut said they attacked government positions close to the border on Friday morning, and that four government soldiers and one insurgent were killed in the fighting.
(Additional reporting by Maria Golovnina in Tripoli, writing by Silvia Aloisi, editing by Tim Pearce)
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