Officials say nine protesters were killed when police opened fire on demonstrators demanding the closure of a south Indian copper plant.
Thousands of protesters turned out on Tuesday amid months of rallies against the Sterlite copper smelting plant, which demonstrators say has polluted the area's groundwater.
Media reports from the town of Tuticorin say that pushing and shoving between protesters and police turned more violent on Tuesday, with demonstrators pelting police with stones, overturning cars and setting fires.
The top official in Tamil Nadu state, K Palaniswami, said police “had to take action under unavoidable circumstances to protect public life and property".
Sterlite Copper has requested government approval to expand its immense plant in Tuticorin.
Sterlite is a unit of the firm Vedanta Ltd.