The party workers were inside the office of ward 18 councillor A Puja at Nayakhuli locality when the miscreants hurled a bomb and fired indiscriminately.
Five persons including Puja's husband Srinu Naidu were injured in the firing, police said.
While Naidu and another injured were taken to a Kolkata hospital, the three others were admitted to Midnapore Medical College and Hospital.
Investigation is on and no arrests have been made so far, police said.