A man has been arrested on suspicion of arson with intent to endanger life after a fire forced an evacuation of part of Leeds General Infirmary.
Police were called just before 18:00 GMT on Thursday after reports of a man armed with a screwdriver threatening to start a blaze.
Social media posts showed a large number of police and fire vehicles with reports of smoke in corridors.
West Yorkshire Police said the 43-year-old man made threats and caused damage.
In a statement, police said they were called to Leeds General Infirmary "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 area was evacuated and officers attended, located, and arrested a man."
Det Insp James Entwistle added: "This was a very serious incident which caused significant damage to the hospital.
"Luckily no one was injured and everyone in the hospital was safely accounted for.
"I would like to appeal to anyone who was impacted by the fire in either the A&E or X-Ray department between 5 and 6pm (1700-18:00 GMT) to come forward with any information to assist our inquiries".
West Yorkshire Fire & Rescue said: "Six fire engines attended, two each from Leeds and Hunslet fire stations, one each from Moortown and Stanningley plus the aerial appliance from Bradford Fire Station.
"Crews used eight breathing apparatus, four hose reels and a PPV [positive pressure ventilation] fan to tackle the fire. They also assisted hospital staff with an evacuation.
"No one was injured and the fire did not spread."
The fire service said that crews left the scene shortly before 21:30 GMT.
A spokeswoman 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.