- The van drove into people sitting at tables outside a restaurant
- Police said the driver took his own life after the crash
- Police asked people to avoid Muenster city centre
The vehicle ploughed into people sitting at tables outside the Grosser Kiepenkerl restaurant, which is popular with tourists.
"At 15:27 (1327 GMT), a vehicle drove into the outside area of the restaurant ... three people were killed, 20 injured, and six of those seriously injured," police spokesman Andreas Bode said, adding: "The perpetrator killed himself in the vehicle."
It was not immediately clear whether the perpetrator was among the three killed, or whether his death took the number of dead to four. A police spokeswoman said separately that there were at least three people dead.
A security source said the perpetrator was probably German.
Bode said the identity of the perpetrator was not yet clear. Investigators were looking at the possibility that other suspects fled the scene, though they had no evidence that this was the case, he added.
"It is far too early to speak of an attack," Bode said. "We have cordoned off the area widely. The crime scene investigators are checking out the crime scene, trying to identify, investigate and secure traces. That is our current task."
The incident came one year to the day after a truck attack in Stockholm that killed five people.
"I am shocked by the news from Muenster," said Andrea Nahles, parliamentary leader of the Social Democrats, junior partner in Chancellor Angela Merkel's ruling coalition.
"My thoughts are with the victims and their relatives," she added. "I hope that our authorities can quickly clarify the background to this incident and wish the local forces much strength for their work."
Government spokeswoman Ulrike Demmer tweeted: "Awful news from Muenster. Our thoughts are with the victims and their relatives."