According to police and witnesses, a group of 15 to 20 people arrived on eight to 10 motorcycles around 11pm and broke the locks of the office, located at Alorupa Mor in the district headquarters.
The arsonists reportedly looted valuables from the premises before setting it ablaze.
Local residents, concerned that the fire might spread to adjacent shops, rushed to the scene and managed to bring the flames under control.
Alamgir Hossain, General Secretary of the Lalmonirhat municipal unit of JaPa, said the office had already been engulfed in flames by the time party members arrived after being alerted.
While the attackers have not yet been identified, the matter has been reported to senior party leaders, reports UNB.
Tensions continued to grip the area following the incident.
Nur Nabi, officer-in-charge of Lalmonirhat Sadar Police Station, said police responded promptly after receiving information and have launched an investigation to trace the attackers.