Police have sealed off Harbour Road in Pagham. A cordon was set up on local roads and residents have been advised by police to stay inside and lock all doors and windows whilst they deal with the incident.
Residents have described seeing a black Land Rover in the road with armed officers being deployed in the road along with ambulance nearby.
Emergency services were called to a property in Harbour Road, Pagham at around 4.10pm on Sunday (28 August) following reports a 72-year-old man was using threatening behaviour and armed with a gun.
A woman known to the man was also in the house, but she has now left and is safe and well.
There are no reported injuries.
The 72-year-old man remains in the house alone.
A police cordon has been set up around the house and the road has been closed.
Officers, including firearms officers, remain at the scene while negotiations continue.