Karnataka had stopped release of water to Tamil Nadu, citing low storage in the K R S reservoir, yesterday.
Besides Thanjavur, the activists also blocked trains at different places including Tiruvarur and Mayiladuthurai, police said.
Pandian said the Central government should declare the Krishna Raja Sagar and Kabini dam areas as "disturbed area" and strict action should be taken against those who prevented release of water or agitated against release.
He said that all parties should unite and fight for the rights of the Cauvery water.