The conservative club housing the office has been evacuated and forensic officers are on the scene.
A suspicious substance found at the constituency office of British immigration minister Caroline Nokes in southern England is not hazardous, police said on Tuesday.
Romsey Town Council tweeted: "Due to an ongoing incident in the Market Place #Romsey involving all blue light services, there are roads obstructed by emergency service vehicles".
Hampshire Police said in a statement: 'We have been called to Market Place, Romsey, following the discovery of a suspicious substance. "Please avoid area for time being".
Police along with members of the ambulance and fire services, wearing full protective clothing, were seen going into the building.
Ms Nokes has been in the news in the last few days over the Windrush crisis. Home Secretary Amber Rudd was quizzed in the Commons about the potential deportations, however she told MPs she was not aware of any specific cases.