A tight security ring has been thrown around the city for Independence Day.

The Lalbazar police headquarters has received four threat letters in the past week warning of blasts on or before I-Day. Though the letters appear to be hoax, the cops are taking extra precautions.

Special forces, including commandos and special action force personnel, are keeping a close eye on suspicious activities in the city. “More than 5,000 policemen will be put on duty for I-Day. Sleuths of special branch are also engaged in collecting intelligence,” said Vineet Goyal, the deputy commissioner of police (headquarters).

Officers of the Heavy Radio Flying Squad (HRFS) and the Radio Flying Squad (RFS) have started patrolling the city streets from Thursday.

The police have asked the Border Security Force (BSF) to keep a close watch along the India-Bangladesh border. “There is round-the-clock vigil on the highways at Dankuni and Kona Expressway as they are the arterial roads linking the city to other states,” said a detective department source.

Security has also been beefed up at the airport. All vehicles entering the airport are being checked twice, once by the police near gate No. 1 and then by the CISF before entering the canopy. “All hand bags are being frisked manually after the X-ray screening,” said K.N. Tripathi, the CISF commandant at the airport.

Railway police are checking all long-distance trains with sniffer dogs. “Bags are being checked with hand-held metal detectors. Security on platforms has been tightened,” said a railway police officer.

Ashok Biswas, the additional superintendent of police, Salt Lake, said security has been tightened for the past two days. “Bomb squads and sniffer dogs have been deployed in Salt Lake, specially Sector V.”