Officers were called to Salford Royal Hospital in Greater Manchester at 9.20pm yesterday, after a man made threats towards staff, police said.
A weapon was recovered and the man was detained on suspicion of possessing a firearm and false imprisonment. A woman was also arrested on suspicion of possessing a firearm. Both remain in custody.
Greater Manchester Police said no one was injured during the incident, and inquiries were continuing.
More top news
Police have revealed a pair of sandals they have had made similar to the pair Ben is thought to have been wearing when he disappeared
The hospital was last attacked just three days ago, something that UN Secretary General Ban Ki Moon denounced as a war crime.
Former UKIP leader Nigel Farage has flown to the US to help coach presidential candidate Donald Trump ahead of his next debate.