One photo journalists Dulal Deka and a TV reporter of a local channel Gagan Jyoti Saharia besides three police personnel - Prabodh Nath, Gauri Sharma and Pabitra Saikia were injured in the blast, they said.
All the injured have been taken to the Mangaldoi Civil Hospital for treatment.
Two police vehicles stationed inside the complex also got damaged due to the blast.
Police suspect that ULFA militants threw the grenade.