Police closed a Brighton road while armed police talked to a man inside a house there following reports of a knife.
Sandown Road off Elm Grove was blocked by a police car while officers attended the ongoing incident.
A Sussex Police spokesman said: “We were called at 11.25am by a member of the public who reported a man was injured and was carrying a knife. Armed police were called because of the reports of a weapon.
“Officers spent some time speaking to the man inside an address on Sandown Road
“At 2.15pm officers entered the home because they believed the man was injured and that he needed medical attention.
“The man was detained under section 135 of the Mental Health Act and has been taken to hospital for treatment.”