Dozens of football fans scuffled and threw chairs after a live screening of Poland's 2-1 World Cup defeat to Senegal at a Belgian outdoor cafe.
A police spokesman told Reuters on Wednesday that one person was hurt in the incident, in the port city of Antwerp, but no arrests were made.
Local newspaper Gazet van Antwerpen quoted police as saying some 40-50 people had been involved.
Bar manager Johan Peeraer told the paper that around 100 Poles and 30 Senegalese supporters had been watching the game when things got tense.
"A bit of an argument started outside and it got out of hand and people started throwing chairs," he said.
Disturbances around the tournament have been rare, a week into the month-long event.