MDMK leader Vaiko threw his weight behind the protest and addressed the protesters at Ininthakarai, about five km from Koodankulam in the district, as the protest intensified.
Around 10,000 fishermen from various hamlets in and around Koodankulam joined a core group of more than 100 fellow fishermen of Idinthakarai whose indefinite fast on the issue entered the third day today, police said.
More than 4,000 people had observed a fast yesterday protesting against the plant. Vaiko said he had been opposing the plant and had raised the issue even in Parliament in 1988 when the agreement for it was signed between Russia and India.
He said the local people were apprehensive about the project especially after the Fukushima nuclear power station disaster in Japan following an earthquake in March.
The project envisages building of two 1,000 MW VVER type Russian reactors by the Nuclear Power Corporation of India.
KNPP officials last month said the hot run of the first reactor, i.e., testing with dummy fuel, has reached the final stage and commercial power production would begin in December.
Protests against the project was stepped up on August 15 when a meeting of the local village sabha at Idinthakarai resolved that the plant should not be commissioned.