Two employees among other protesters set themselves on fire and were badly burnt before the flames were doused, Workers' Union president Harkit Baitha told reporters.
District Superintendent of Police Jitendra Rana, however, did not rule out foul play by fellow protesters.
An FIR has been lodged against 20 people and seven out of them have been arrested so far, the SP told PTI.
Rana said District Magistrate Anupam Kumar was scheduled to hold talks with the protesters over their demands.
More From This Section
Five policemen were injured in the violence, the SP said adding, the police had to fire three rounds in the air and lob tear gas shells to disperse the unruly crowd.
The two badly burnt workers have been rushed to Patna Medical College and Hospital for treatment, Rana said.
The workers were demanding payment of their arrears and also reopening of the sugar mill at the earliest.