Lincoln County Hospital was put on lockdown on the evening of Monday, July 25, after a person began making threats of violence, eyewitnesses said.
The incident, which occurred some time after 8pm, saw many sections of the hospital closed, with doors locked for patients, visitors and staff.
Reports are surfacing on social media that the person involved, thought to be a patient, was carrying a weapon.
This has not been confirmed by the hospital or Lincolnshire Police.
It is not known if anyone was injured in the incident.
It is understood that the hospital was reopened again at around 9.30pm.
One person who had tried to enter the hospital told The Lincolnite the doors have been locked. They said they believed a patient had been threatening to hurt others.
Another said they were unable to leave and that a man inside had become violent.
Were you at the scene? Reach our news team on email@example.com or 01522 837217.