Lalbazar: Traffic sergeants in Calcutta have been told to record every prosecution they make with the body cameras attached to their uniform.

The use of body cameras had until now been left to the discretion of the sergeants, who would usually switch them on only when they thought a particular prosecution was likely to invite a dispute or controversy.

The instruction to record "all" prosecutions came after an incident in front of Lake Market last Saturday involving the vehicle of a senior bureaucrat.

The move is a double-edged sword: traffic cops would need to be extra cautious about their own conduct while capturing the response of the offender as a piece of potential evidence.

Sources said the instruction to have every prosecution on camera was "passed down the line" through the traffic police's official WhatsApp group a few hours after the incident in Lake Market.

A sergeant had allegedly slapped the driver of a vehicle ferrying the bureaucrat's wife after the two had had an argument over the car being parked in a double line.

Senior officers in Lalbazar, the city police headquarters, said footage from the sergeant's body camera shows him slapping the driver. But what prompted such a reaction from the sergeant is not on camera.

It could not be immediately confirmed why the earlier part of the conversation between the sergeant and the driver was not captured by the body camera, sources said.

Calcutta police had introduced body cameras in September 2016. Each of these cameras save a video clip every three minutes. In dim or low light, the infrared LED mode is activated automatically so that there is enough light to record.

"It's a two-way system meant to bring transparency," said a senior officer.

Officers of the rank of assistant commissioner of police are usually in charge of assessing and preserving the footage collected from the traffic guards.

The police use a similar mechanism to counter complaints of wrong prosecutions in cases of signal or stop-line violations at intersections where CCTVs are installed.

Bidhannagar City Police uses body cameras too and officers are required to keep them on whenever they are on duty.