Protesters in the southern Indian city of Bangalore in Karnataka state have attacked shops and set fire to vehicles in a long-running dispute about water.
They were angry at a Supreme Court ruling ordering Karnataka to share more water with neighbouring Tamil Nadu.
Karnataka must release 12,000 cubic feet of water per second from the Cauvery river until 20 September.
Both states say they urgently need the water for irrigation and a battle about access to it has raged for decades.
The violence in the technology hub closed many offices and much of the public transport system.
Police have imposed an emergency law that prohibits public gatherings, and more than 15,000 officers have been deployed.
Reuters reported that Tamil Nadu registered vehicles were being singled out by protesters and pelted with stones.