Dutch police said a gunman opened fire in a classroom at a university hospital campus in Rotterdam and a nearby house on Thursday causing multiple deaths.
A 32-year-old suspect was arrested after police said on social media that shots had been fired at the Rotterdam Medical Centre and a home. They also said fires had broken out at both locations.
It was unclear how many people were hurt at each location.
Officers were entering the premises of the hospital university in search of possible victims or people hiding, Dutch news agency ANP reported.
Videos posted online showed police instructing students, some wearing medical gowns, to run outside as heavily armed arrest teams arrived at the scene. One video showed a man in handcuffs wearing what appeared to be camouflage pants.
Two hours later police said there had been multiple deaths and that victims' family members were being informed.
"This is a very dark day," caretaker Minister of Justice and Security Dilan Yesilgöz was quoted by newspaper de Telegraaf as saying in comments made in Brussels.
Police said there were no indications of a second shooter. A press conference was scheduled by local officials.