The renewed protest comes two days after Prime Minister Manmohan Singh assured a multi-party delegation including the anti-KNPP activists that an expert group would be set up to allay their safety concerns.
Slamming Singh's letter to Chief Minister Jayalalithaa seeking her support to ensure timely implementation of the Indo-Russian project, protesters said it only showed the Centre was not concerned about the safety of people.
The fast at Idinthakarai near Koodankulam in the district was led by co-ordinator of People's Movement Against Nuclear Energy S P Udhayakumar, a member of the delegation that had met Singh.
Udhayakumar told reporters the fast was only "a beginning of their long struggle" against nuclear power and it would be intensified if the KNPP was not scrapped.
After meeting the Prime Minister, the activists had claimed it was their first victory. But now they are irked by Singh's letter to Jayalalithaa seeking her help to complete KNPP.
Udhayakumar said the contents of the letter, dashed off on the same day when they met Singh, only showed that the "Centre is not concerned about the safety and security of the Tamils."
Despite the state cabinet's resolution for halting work on the project, the Centre had chosen to go ahead with it. "This is painful," he said.
People, including fishermen, from 13 villages participated in the fast on Sunday.