An anonymous letter received by some news organisations in Chennai on Thursday warned of terror strikes at key city locations, including the airport, the central railway station and the US Consulate General complex.
The letter, sent by post, said Chennai will face the "biggest terror demonstration" and places such as the airport, Police Training College and the Officers' Training Academy (OTA) of the Army will be attacked.
The sender also described the modus operandi for each of the targets in a tabular column.
It also gave the police a "clue" when it said the "plan" is being made from a specific locality in the city but did not mention the date for the attacks.
The letter has been handed over to police, who believe it could be a prank.
However, the city police commissioner said "all standard operation procedures (to handle such situations) will be carried out".