Three protesters were killed on Sunday at Paramakudi in the district after police fired at stone-pelting dalit agitators following reports that their leader had been detained, the security agency said.
15 policemen, including DIG Sandeep Mittal, were injured in the stone-pelting as the mob set ablaze vehicles.
Violence broke out after news spread that John Pandian had been detained at Tuticorin en route to Paramakudi to pay homage on the death anniversary of dalit leader Imanuel Sekar.
Police used tear gas and lathi-charge before firing at the crowd at Paramakudi, 40 km from here.
According to initial reports, 15 vehicles including a police vehicle and buses were set ablaze.
The DIG was injured seriously, officials said.
The violence spread to some adjoining areas and traffic has been blocked in 15 places.
A police officer said they had sought additional reinforcements to quell the violence. Bus services to most parts of Ramanathapuram district have been suspended.
Police sources said Pandian had been detained at Vallanadu in Tuticorin as a precautionary measure.