Unidentified men threw acid on a 25-year-old constable at a police station in Uttar Pradesh's Varanasi district on Wednesday, an official said.
The incident took place inside the Adampur police station when some men assaulted Dinesh Sonkar and later threw acid on him before leaving the place.
"Sonkar's condition is stated to be critical. We are yet to establish any reason behind the acid attack. Efforts are on to nab those involved in the attack," police inspector Dalbeer Singh told reporters in Varanasi, about 300 km from Lucknow.
"The incident has definitely brought security lapses to the fore. It's a matter of enquiry as to how the men managed to barge inside the police station, where officials are supposed to be present round-the-clock," Dalbeer Singh said.