Police said that three persons, including two local Trinamool Congress (TMC) leaders, have been detained for questioning in connection with the incident.
The victim, identified as constable Sandip Saha, had to be rushed to a local health centre, from where he had to be shifted to a hospital in Kolkata when his condition deteriorated.
People familiar with the matter said that Saha was alone in the police camp when the incident happened. It is suspected that the attackers had come on a motorcycle.
Sandeshkhali in North 24 Parganas district had hit the headlines after a team of Enforcement Directorate (ED) officials, who went to search the house of local TMC strongman Shahjahan Sheikh, were attacked by a mob on January 5.
In February villagers, mostly women, hit the streets demanding the arrest of Shahjahan Sheikh and his aides over allegations of sexual assault and land grabbing.
He was arrested on February 29 on the orders of the Calcutta high court and was later handed over to the Central Bureau of Investigation (CBI).