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 delegationthat had met Singh.
Udhayakumartold 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.
Aftermeeting 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.
Udhayakumarsaid 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."
Despitethe 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 13villages participated in the fast on Sunday.