Motorcycle speedway fans attacked police with stones overnight in Poland, violence that began after a fan was accidentally run over and killed by an unmarked police vehicle.

The violence broke out in the early hours of Monday in Zielona Gora, a city in western Poland. The speedway fans broke shop windows, demolished a fuel station and wrecked several police cars.

Prosecutor Alfred Staszak said two police officers were injured in the violence; one suffered a broken jaw after being kicked in the face and the other had a cracked skull. Nine people were arrested.

Prime Minister Donald Tusk, who has been traveling the country as he campaigns ahead of elections next weekend, returned to Warsaw to consult with police and other security officials over the matter. He said he planned to meet with family members of the man who was killed to express his condolences to them, and also show his solidarity to the injured police officers.

"The most important thing now is to ensure the stability of the state," Tusk said, adding that he hoped the incident would not become an issue in the election campaign.

National elections are taking place Sunday. Tusk's centrist Civic Platform is hoping to hold onto power but has seen its comfortable lead in opinion polls eroded recently.

Recent polls show it is only a couple of points ahead of its main rival, the conservative nationalist Law and Justice party.

Authorities have made a priority of fighting sports-related violence as the country prepares to co-host the Euro 2012 football championship.

The overnight rioting occurred after the local motorcycle speedway club Stelmet Falubaz defeated rival Unia Leszno for the national championship.