A driver who ploughed his car into crowds on a street Friday in the Polish city of Szczecin injuring 19 people, including six children, was undergoing psychiatric treatment, police said, ruling out terrorism.

Early investigations into the incident "showed that this was a person who has been undergoing psychiatric treatment for the past four years," police deputy superintendent Marek Jaszczan told reporters.

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He revealed police and prosecutors were seeking to "determine the motives of this person" but that "a terrorist act" could be ruled out.

"The perpetrator of this incident is a 33-year-old Polish citizen residing in Szczecin," Jaszczan added.

Witnesses said the car arrived a high speed and hit people waiting for a tram in the centre of the northwestern city.

The motorist continued for another kilometre (0.6 miles) before crashing into three other vehicles, injuring the drivers, they added. The man was then detained.

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"Among the injured there are six children and two of the injured are in critical condition," provincial governor Adam Rudawski told reporters, adding 15 were taken to hospital.

Szczecin police spokesman Pawel Pankau said there was no indication that the suspect had been drinking alcohol or consuming drugs.