Members of the public have been warned to stay away from the street as officers work to control the situation.
Police negotiators have been called in.
A Sussex Police spokesperson said they had been called to Portland Road this morning after concerns for a man in a property.
They said: "The man has thrown items from the window and a section of the road has been closed while officers try and speak to him.
"The incident is contained within the flat and there is no threat to members of the public but they are asked to stay away while officers are on scene."
The road between Rutland Road and School Road is currently closed.
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