Prime Minister Mark Rutte said he was "deeply concerned" and that crisis talks were to be held in response to the incident. Dutch radio said security had been increased at the seat of the government in The Hague.
"Several shots were fired in a tram and several people were injured. Helicopters are at the scene and no arrests have been made," said police spokesman Joost Lanshage.
Police could not immediately provide further details about the shooting incident and could not say how badly hurt the victims were. The victims’ identity was unknown.
Local broadcaster RTV Utrecht quoted one eyewitness as saying he had seen a woman lying on the ground amid some kind of confrontation. Several men ran away from the scene, the witness said.
The Utrecht police said The October 21st square, a tram station stop outside the city center, had been cordoned off as emergency services were at the scene.