The 27-year-old man from Leeds is being held at a police station in West Yorkshire and being interviewed by officers.
Wards on the Gledhow wing of St James’s Hospital in Leeds were evacuated after the suspect was detained at around 5am on Friday.
He was initially arrested on suspicion of firearms and explosives offences, but police later said he was re-arrested on Friday evening under Section 41 of the Terrorism Act 2000 on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Counter Terrorism Policing North East said a warrant of further detention was granted on Saturday by Westminster Magistrates’ Court, giving officers until Friday to hold and further interview the man.
Police said there are “extensive inquiries ongoing to establish the full circumstances, and explore any potential motivation”.
Army specialists and a bomb disposal unit attended the scene on Friday and a cordon was put in place as a precautionary measure.
Detective Chief Superintendent Jim Dunkerley, head of Counter Terrorism Policing North East, said: “We continue to treat this as an isolated incident and there is still no evidence to suggest there is any heightened or ongoing risk to the public.
“Nevertheless, the UK threat level remains at substantial and we would always encourage the public to be vigilant and to report any suspicious activity or behaviour to police in confidence on 0800 789 321 or at gov.uk/ACT.”
St James’s University Hospital is famous as Europe’s largest teaching hospital, according to the trust website.
It is affectionately known as “Jimmy’s” and was the subject of a television show of the same name which first aired in 1987 and followed goings-on at the busy hospital.
