A footballer was injured when three explosions went off near the Borussia Dortmund team bus.
The bus was near players’ hotel, carrying them to their Champions League quarter-final and player Marc Bartra was injured, the club said.
Police said they were working on the assumption “serious explosive devices” were used.
Photos showed glass of the bus’ windows had been smashed by the explosion.
Police initially tweeted there had been an “explosion” near the team’s bus.
Bartra has been taken to hospital. He is not badly injured and was hit by glass from the bus’ windows, according to reports.
The match against Monaco was quickly postponed until tomorrow evening. Tens of thousands had travelled to the German city for the game.
The other Champions League match, between Juventus and Barcelona, is going ahead in Turin.
The other Dortmund players were unharmed, the club tweeted.
Bartra, 26, is a centre-back for the team and a Spanish international.
In the stadium where the match was due to take place, Monaco fans were chanting “Dortmund! Dortmund!” in solidarity.