Delhi Police ramped up security at railway stations, bus terminuses, the airport, popular markets and places around vital installations in the city, a senior official said.
Delhi Police's counter-terrorism unit Special Cell and the Crime Branch have also been roped in, the official said adding, local police have been alerted and a tight vigil put in place at the porous borders of the city.
While three of the planes had already departed by the time the anonymous call about the bomb threats was received, the fourth aircraft, which was bound for Chennai, was diverted to Nagpur, sources said.
Another plane, also bound for Chennai which was preparing for take off, was grounded and undergoing checks by the security agencies, a senior police official said.