The Coast Guard aircraft hovered over the sea and its ships kept a vigil off nearby Idinthakarai as the villagers, including women and children, from Kudankulam and nearby fishing hamlets walked into the sea for the show of strength.
With black flags fluttering in the backdrop, the protesters said they were prepared to sacrifice their lives to protect their livelihood and ecology through the 'jal satyagraha', taking a leaf from a similar protest in Madhya Pradesh.
"What we are observing is Jal Sathyagraha - peaceful demonstration in the sea," said anti-nuclear activists who have taken the example of villagers of Khandwa district in Madhya Pradesh demanding land as compensation and reduction of Omkareshwar Dam recently.
Though the protest was intended to be from 10 am to 4 pm, the activists said they would continue it indefinitely on a relay basis.
Officials said Coast Guard aircraft and ships had been deployed to monitor the entire coastal area even as more than 4,000 police personnel, supported by Rapid Action Force, continued to maintain a strict vigil, having almost sealed the entire Kudankulam town to bar entry of outsiders.
Police said they were monitoring the human chain protest from a distance and would not act unless 'provoked extremely'.
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The protest in the sea was held even as the Peoples Movement Against Nuclear Energy leader S P Udhayakumar, who has been spearheading the over a year-long stir, remained elusive with police searching for him.
Udayakumar had announced he would surrender before police on Tuesday night but later did a somersault citing people's sentiments against it. (MORE)