The clash erupted in Agatpur village of the district last evening when one of the groups was carrying a procession, which was allegedly opposed to by the other group, police said.
While one Chandreshwar Lal (30) died during treatment at the Primary Health Centre, 12 others, including Singhwara Station House Officer Rakesh Kumar were injured, Deputy Inspector General (DIG) Uma Shankar Sudhanshu said.
"The administration is treating the death of Lal as a natural one, but the villagers are insisting that he died during the clash. We have sent the body to Darbhanga Medical College and Hospital (DMCH) for post-mortem to ascertain the cause," the DIG said.