The letter, which was written in Tamil, has been signed by "Kudankulam Makkal Nanban" (Friend of Kudankulam people).
"The three-tier security arrangement is only for the plant and not for the officials," threatened the letter which was posted from Anna Salai in Chennai.
The letter was sent just a day after Atomic Energy Commission Chairman Ratan Kumar Sinha's statement informing about the first 1000 MW unit of the Indo-Russian project which would be commissioned in about two weeks.
Following an official complaint, Tirunelveli Superintendent of Police Vijayendra S Bidari asked Tamil Nadu police to file a case and start an investigation into the matter.
A joint security drill by Coast Guard and state Marine police has been organised at Kudankulam coast on Saturday, Jan 5.
The commissioning of the plant initially was scheduled for Dec 2011. However, officials postponed their plans citing tremendous protests by local people and fishermen of the region against the nuclear plant.
People's Movement Against Nuclear Energy agitators held several protests at different times claiming that the nuclear plant can harm the safety of the people who stay near the plant.