Benghazi Medical Centre said the dead were mostly soldiers and civilians fighting alongside the army in the central district of Al-Majouri.
At least 52 people have been killed since forces of retired general Khalifa Haftar, backed by army units, on Wednesday launched what he called an operation to "liberate" Benghazi from Islamist militias, according to hospital figures.
An AFP correspondent in the eastern city said today's clashes were the fiercest since the start of the operation.
Al-Majouri is home to the leader of the Ansar al-Sharia jihadist group, Mohamed al-Zehawi, and many of his men.
Air raids also struck Islamist targets in the districts of Al-Lithi, Al-Massaken et Bouatni, southeast of the Mediterranean city, witnesses said.
Islamist militias have seized control of large parts of turmoil-gripped Libya since a 2011 uprising against long-time leader Moamer Kadhafi.