The incident on the train happened at around 11pm. A passenger telephoned the police to report that a group of between 10 and 15 people were letting off bangers inside the train.
A little later, the group was spotted setting off fireworks at Leuven railway station.
The police detained the group of youths as it made its way to the Grote Markt. They had more than 200 bangers and several rockets with them. The fireworks were confiscated.