Patients and staff had to be evacuated from Leeds General Infirmary on Thursday night after a small fire broke out.
Emergency services rushed to the scene amid reports an A&E corridor filled up with smoke.
West Yorkshire Police said a man has been arrested in connection with the fire.
Officers were called to the hospital at about 17.54 to a report of a man in possession of a screwdriver who was causing damage in a unit, had made threats, and was attempting to start a fire.
The suspect remains in custody while enquiries are ongoing.
Images show several police vans and fire trucks outside the hospital.
The fire has been contained and nobody is thought to have been injured.
A man who used to work in the building told the Yorkshire Evening Post: ‘I heard that the smoke was filling up the corridors from A and E up to ICU and Theatres (one level up).’
Reporter at the scene Joe Cooper tweeted that he was told three fires were set off deliberately but this has not been confirmed by emergency services.
A spokesperson for Leeds Teaching Hospitals NHS Trust said: ‘We can confirm that there was a small fire at Leeds General Infirmary this evening.
‘The fire was extinguished quickly and nobody was injured.
‘Staff and patients were evacuated from the building for a short period and police investigations into the cause of the fire are ongoing.
‘We would like to thank our staff and the emergency services who responded quickly to the incident.’
This is a developing story, more to follow