Tapan Haidar Shun, 50, general secretary of Santhia Awami League, and Marufa Khatun, 23, a pedestrian, sustained bullet injuries during exchange of fire between the groups while Sub Inspector Saiful Islam, 50, and Constable Shohag Mahmud, 30, were injured by hurled bricks.
They were admitted to the upazila health complex.
Identities of two other injured could not be known yet.
Witnesses said there was a rivalry between the followers of Tapan and Mirajul Islam Miraj, mayor of Santhia municipality and also a member of the upazila AL, over recently concluded Zila Parishad elections.
Tapan filed a case against Miraj after some miscreants set fire on his personal office on December 28, the day of elections.
On Sunday morning, the clash ensued when the processions, brought out by AL activists of both groups in protest of setting fire to Tapan’s office and filing the case against Miraj, reached in front of Islami Bank in the municipality area.
Later, police reached the spot and brought the situation under control firing tear shells.