South East Coast Ambulance Service (SECAmb) confirmed it was called shortly before 11.50am to reports a person had fallen from a bike on Arun Parade near Harbour Park.
A spokesman for SECAmb said: "Ambulance crews attended the scene and were joined at the scene by the air ambulance service.
"The male patient was assessed and treated at the scene before being taken to Royal Sussex County Hospital in Brighton in a serious condition."
Sussex Police confirmed it assisted at the scene after reports of a 'medical incident'.
A spokesperson said: "A man in his 60s was left in the care of medical crew and police were stood down."
Video courtesy of Jason Hughes.
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