* Heaviest bombardment since fighting started - rebel
* 25 dead, including children - doctor
* Residents say shelling continues; rebels claim victory
* Misrata is last big rebel stronghold in west Libya
(Updates with reports of further shelling)
By Tarek Amara and Mariam Karouny
TUNIS, March 18 (Reuters) - Forces loyal to Muammar Gaddafi shelled the rebel-held city of Misrata with tanks and heavy artillery on Friday, several hours after Tripoli announced a ceasefire following a United Nations vote for a no-fly zone.
"There is still fighting going on," said Tariq, a doctor in Britain in close contact with colleagues in Misrata. "I spoke to my doctor contacts in Misrata ... they can hear some shelling."
The doctor said he spoke with Misrata around 1640 GMT, four hours after Foreign Minister Moussa Koussa said Libya had decided to halt all military operations in the country to protect civilians and comply with the U.N. resolution.
A doctor in Misrata, the last big rebel stronghold in western Libya, earlier said at least 38 people had been killed in the assault launched on Friday morning, in the wake of the U.N.'s security vote late on Thursday.
"Gaddafi's forces are bombing the city with artillery shells and tanks," 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 the ceasefire announcement and a rebel claim the attack had been defeated.
"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 earlier said the insurgents had beaten back the attack, despite the heavy weapons used by Gaddafi's forces and the fact the coastal city of 300,000 has been under siege for days.
KILLINGS "INDISCRIMINATE"
"The fighting is on the outskirts, not in the city centre," said the doctor in Britain. "Yesterday there was celebration after the U.N. vote, even by ordinary people, they don't want the regime any more."
Several residents said government tanks were closing in on the centre of the city, about 200 km (130 miles) east of Tripoli.
Tariq, the doctor in Britain, had earlier reported continued shelling in the city after the ceasefire announcement and 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 mosques, schools and residential buildings, they said. The reports could not be independently verified because journalists are barred 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 and Tom Heneghan; Editing by Sophie Hares)
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