The incident occurred at around 6 pm when a group of policemen in civilian attire had gone to the Hojai railway station area on a secret operation, police said.
Some gunmen fired at the police team led by Sub-divisional Police Officer (SDPO) of Hojai, Jyoti Ranjan Nath.
While the SDPO escaped unhurt, another policeman Litumoni Kalita sustained bullet injuries.
The injured cop was admitted to Guwahati Medical College,where his condition was stated as serious.
An operation had been launched to nab the gunmen, police said.