Police negotiators were last night locked in an armed stand-off with a man at a house in a small rural village.
Lincolnshire Police said a "significant number" of officers were at the property in Sutton St James, Fenland, including armed police and specially trained negotiators.
A force spokesman said a "sterile cordon" had been imposed in "four or five" streets close to the incident in Chapelgate, leaving residents housebound or unable to return to their homes if they were out when the siege began.
Police have been at the scene since around 11am and an officer has suffered minor injuries in the incident.
The force refused to go into detail about the situation at the house, but sources confirmed that a man armed with a gun was inside the property.
Local people not already at home have been asked to go to nearby houses of friends or relatives while officers deal with the incident or to make their way to Sutton St James Primary School in Bells Drove.