Photos of the explosion site show twisted metal rods and chunks of concrete strewn on the ground in front of the TMC office, as policemen walked about the compound to gather evidence.
The police are checking what caused the blast. The injured people have been admitted to the hospital.
The injured were admitted to a hospital and the police have cordoned off the area.
The sources said it was also not known whether a crude bomb had exploded or a gas cylinder inside the party office led to the incident.
Pradyut Ghosh, TMC MLA of Narayangarh, said that the cause of the explosion could not be ascertained till noon and the police were looking into it.