"The gunning down by the police of nine people in the #SterliteProtest in Tamil Nadu is a brutal example of state-sponsored terrorism. These citizens were murdered for protesting against injustice. My thoughts and prayers are with the families of these martyrs and the injured," he tweeted.
Earlier in the day, nine people were killed and 65 injured as clashes erupted between agitated protestors and police officials in Tuticorin.
Vehicles and properties were also vandalised during the protest.
Protestors demanded a ban on Sterlite Industries, in wake of the pollution emitted by the plants.
Meanwhile, Tamil Nadu Chief Minister E Palanisamy announced compensation of Rs 10 Lakh each for those killed in the protest, and Rs 3 Lakh each for those injured.
Furthermore, Palanisamy announced that government jobs would be provided for the kin of the deceased, and an inquiry commission would be set up for an investigation into the matter.