Firearms officers have been negotiating overnight with a armed man at a house in Pagham in Sussex.
Police were called to reports a 72-year-old man was using threatening behaviour and had a gun.
A woman known to the man left the house safely. A police cordon is currently in place.
More top news
Coprorals Matthew Hatfield and Darren Neilson died of their injuries after an incident at an Army live-firing range
Detectives are investigating the murder of Dawn Sturgess, 44, and the poisoning of her partner, Charlie Rowley, 45.
Labour sources have also indicated they will oppose a plan for the Commons to rise for summer recess on Thursday instead of Tuesday July 24.