Key installations, bus and railway stations were on high alert one day after a group of gunmen attacked an air force base in north-western India.
Several trains were delayed in New Delhi bomb squads and canine units inspected a high-speed train after a bomb alert.
Mopping up operations were going on at the Pathankot air base in Punjab state, after three airmen were killed and five wounded in a pre-dawn attack by about a half dozen gunmen, air force spokeswoman Rochelle D'Silva said.
India Today television said six military personnel had died, and the toll could rise as more bodies were recovered, but that could not be independently confirmed.
Four of the attackers had been killed and their bodies recovered. There could have been more who escaped during the shootout, NDTV reported citing intelligence sources.
Assault rifles, mortars, grenade launchers and Global Positioning System devices were recovered at the scene.
The government said it had intelligence reports that terrorists would try to infiltrate military installations in Pathankot area, and the base was prepared to respond.
The gunmen were detected as soon as they entered and contained in a limited area, away from the technical zone where high-value aircraft are parked, the Defence Ministry said.