A tribunal set up 17 years ago to decide on a dispute over sharing water from the Cauvery river in southern India has announced its verdict.
There have been dozens of meetings to solve the dispute
Tamil Nadu state is to get 419bn cubic feet of water a year. Karnataka will get 270bn - less than half of what it says it needs.
The dispute is more than 100 years old. Karnataka has said that it will appeal against Monday's decision.
Kerala state is to get 30bn cubic feet and Pondicherry will get 7bn.
Security forces in Bangalore, the capital of Karnataka, have been put on high alert to prevent a repetition of anti-Tamil rioting after a 1991 panel verdict that left 18 dead.
Karnataka and Tamil Nadu have both argued that they need the water for millions of farmers in the region.
The Indian constitution defines the Cauvery as an 'interstate' river.
It originates in Karnataka, flows through Tamil Nadu, and parts of Kerala and Pondicherry.
The dispute over its waters originated in the 19th Century during the British rule between the then Madras presidency (modern day Tamil Nadu) and the province of Mysore (now Karnataka).
Bangalore police made at least 700 preventive arrests before Monday's verdict was announced.
Violence in Bangalore after the panel's 1991 verdict
More than 16,000 police have been deployed to ensure peace.
Bangalore is the technology hub of India and is home to a large number of Indian and foreign multi-nationals.
Bangalore Police Commissioner Achyut Rao told the BBC that hundreds of people have been taken into custody as part of the security measures to prevent a repeat of the 1991 riots.
"We have issued instructions to deal with any law and order situation sternly," Mr Rao said.
Bus services between the two states have been suspended as a precautionary measure and many schools and colleges across Bangalore have been shut.
Karnataka Chief Minister HD Kumaraswamy held an all-party meeting and called for peace in the event of the tribunal order going against the state.
The Karnataka Tamils' Federation - which represents the minority Tamil community - has written to President APJ Abdul Kalam, Prime Minister Manmohan Singh and local officials asking for security for Tamils living in Bangalore and other parts of the Cauvery river basin.
Federation president AP Shanmuga Sundaram said Tamils were wary of a backlash if the order went in favour of Tamil Nadu.
The Cauvery river water tribunal was set up in 1990 after the failure of several rounds of talks between the two states.
Dozens of meetings have been held to find a settlement of the century-old dispute.