Four men have been arrested at a Staffordshire hospital under the Terrorism Act.
Police have cordoned off part of the hospital grounds
Police were called to Stafford General Hospital by staff concerned about the illness of a 28-year-old man and the behaviour of four men with him.
The four, aged 24 to 30, are being held at a police station in the county.
Police said on Thursday afternoon there had been no terrorist incident. They said the man who is very ill has been moved to another hospital for tests.
A spokesman for Staffordshire Police said: "We do not yet know the precise nature of these events at this stage there is no evidence a terrorist incident took place nor of any specific threat to the area."
He confirmed the arrests were made under the Terrorism Act 2000 and said it gave officers the opportunity to investigate "what appears to be a complex set of circumstances".
Two sections of the hospital car park remain cordoned off with police tape, with two police cars parked nearby.
Armed officers arrived at the car park at 2145 BST on Wednesday after being alerted by staff at the hospital.
Martin Yeates, chief executive of the Mid Staffordshire General Hospitals Trust, said access was restricted for a "short period" and ambulances diverted to other hospitals.
"From this morning the hospital has been operating as normal and patients should attend their appointments as normal," he said.
"The hospital operated as normal throughout the night. Hospital patients were unaffected by the police operation."
Buckingham Palace has confirmed a scheduled visit of the Queen to Stafford on Friday will be going ahead as planned.
A spokeswoman said: "As it stands, there is no change to the royal visit and the Queen will be arriving in Stafford tomorrow morning."
Stafford's Labour MP David Kidney said he thought there was "no substance" in the incident and thought it was probably members of hospital staff being over zealous in reporting something suspicious ahead of the Queen's visit.