Police have spent a second night negotiating with a 72-year-old man who remains inside a house in Harbour Road, Pagham.
Emergency services were first called to the property at around 4.10pm on Sunday to a report of threatening behaviour. A woman known to the man was also in the house at the time, but she left shortly after police arrival and is safe and well.
The man, who is on his own and believed to have a gun, continues to communicate with trained police negotiators.
Chief Superintendent Neil Honnor said: "I appreciate this must be a little disconcerting and frustrating for local residents having a continued large police presence outside their homes.
"The safety of the local community is paramount and we are doing all we can to resolve this ongoing incident in a controlled way.
"The road closure and cordons remain in place but residents are still free to come and go as and when they need to with police support."
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Police body cam footage captured the incident in Brighton in June 2013.