An explosion on a tram in the Ukrainian city of Dnipropetrovsk injured nine people on Monday in what police in the European soccer championship co-host said was an accident.

Dnipropetrovsk is not among the cities that host the month-long tournament, but the city had been hit by bombings in April that wounded 30 people.

Interior Ministry spokesman Volodymyr Polishchuk said gunpowder carried by one of the passengers had set off Monday's blast. Registered hunters in Ukraine routinely buy gunpowder to assemble their own rounds.

"The gunpowder ignited for some unknown reason," Polishchuk said. "Either it was very hot or the man dropped it on the floor."

Nine people, including the owner of gunpowder, received minor burns.

Ukraine expects between 800,000 and 1 million foreign visitors during the Euro 2012 tournament. The former Soviet republic, which is jointly hosting the tournament with Poland, is holding matches in the capital Kiev, Lviv, Kharkiv and Donetsk. The final will be held in Kiev on July 1.

On April 27, four bombs planted in trash bins in various downtown locations exploded at short intervals in Dnipropetrovsk, in what prosecutors said was an "act of terrorism".

Ukrainian prosecutors have detained four people on suspicion of being behind the April blasts.

(Reporting by Olzhas Auyezov and Pavel Polityuk; Editing by Alessandra Rizzo)