Pro-Gaddafi forces are reported to have killed 14 rebels who had surged towards the south, east, and west of the city.
Misrata suffered intense bombardment until mid-May, when pro-Gaddafi forces were driven away by rebels.
The latest fighting comes as Nato intensifies its campaign in Libya, calling for the international community to prepare for a post-Gaddafi era.
Earlier a rebel spokesman had told Reuters news agency there had been a fresh government offensive on Misrata.
On Wednesday explosions could be heard to the east of the city as government forces responded with Grad rockets to the rebel offensive.
Rebel fighters told the BBC’s David Loyn in Misrata that they had moved several miles forward of their eastern frontline.
Field ambulances took a stream of casualties for treatment, and traffic built up around Misrata hospital as people responded to an appeal for blood donors, our correspondent adds.
The government has so far not commented on the latest violence.
Meanwhile, Nato ministers met in Brussels on Wednesday to discuss the alliance’s nine-week campaign. Afterwards, Secretary General Anders Fogh Rasmussen told reporters that “real progress” had been made.
Nato action, he said, had “prevented a massacre” in Benghazi in the east – the rebels’ stronghold – and in Misrata.
He added that the UN and other international bodies should start preparing for a post-Gaddafi Libya. “The time to start planning is now,” he said.
