Patients and staff were evacuated from Leeds General Infirmary after a suspected fire on Thursday evening.
Emergency services, including at least four fire engines, rushed to the hospital at about 6pm and a police cordon was quickly put in place.
It's understood one or more fires broke out in the hospital's A&E department.
"I heard that the smoke was filling up the corridors from A and E up to ICU and Theatres (one level up)," a former worker told the Yorkshire Evening Post.
The outlet is also reporting three fires may have been set off deliberately in the hospital, though this has not been confirmed by official sources.
West Yorkshire Police and West Yorkshire Fire and Rescue Service have both been contacted for comment.
It's not yet known if anyone has been injured in the incident, or if the fires were indeed an act of arson.
Footage posted to Twitter shows a significant police presence with ambulances and fire trucks, and a crowd of people watching.
More than four fire engines attended the scene, according to reports from witnesses on social media.
The incident now appears to be under control, with the cordon lifted and people being allowed back inside the hospital.
Local buses are were temporarily unable to travel down Clarendon Way due to the incident at the hospital, with First West Yorks tweeting: "Service 5 towards Albion Street - Due to an obstruction on Clarendon Way the service is currently following the 5A route.
"We are sorry for any inconvenience caused."
Neither police, the NHS trust nor the fire service have responded to requests for comment from the press at this time.
Leeds General Infirmary, commonly known as LGI, is a large teaching hospital which opened in 1869.