Irrigation authorities told PTI that about 5000 cusecs of water was being released from Krishnarajasagar Dam and Kabini Reservoir since Saturday night.
According to sources, Karnataka will ensure teh flow of 9000 cusecs of water to Tamil Nadu.
The decision comes a day after an all-party meeting decided to request Prime Minister Manmohan Singh to review the directive of the Cauvery River Authority, headed by him, asking Karnataka to release 9000 cusecs of water.
The all-party meet decision was taken against the backdrop of the Supreme Court on Friday directing the state to implement the CRA order.
As the water release began, farmers blocked the Bangalore-Mysore highway near Gejjalagere and more than 50 people including Janata Dal-Secular legislator, C S Puttaraju, have been arrested.
The Karnataka State Road Transport Corporation has suspended its bus services apprehending violence.
The police have stepped up tight security in various parts of the district, the epicentre of protests against release of Cauvery water to Tamil Nadu.
On Friday, the Supreme Court had pulled up the Karnataka government for failing to comply with the CRA directive to release 9,000 cusecs of water to Tamil Nadu from September 20 to October 15 and warned that it would have to pass appropriate orders if Karnataka did not fall in line.