Six people died when a car drove into a Carnival gathering in the southern Belgian town of Strépy-Bracquegnies in the early hours of Sunday, the Belga news agency reported.
"At this stage, there are no elements to suggest that the attack had a terrorist motive," prosecutor Damien Verheyen told a news conference. Police denied media reports the car was involved in a high-speed chase.
A further 10 people were seriously injured, the agency reported, citing Mayor Jacques Gobert of the nearby town of La Louvière, 50 kilometres south of Brussels. Around 20 people suffered light injuries.
"A speeding car ploughed into the crowd of people," Gobert said, according to Belga. The driver was intercepted as he tried to get away, he added.
It was unclear whether the incident was intentional. The town has triggered its emergency plan.
The two people driving the car, which was later intercepted, were detained. Police said they were local people in their 30s and were not previously known to police.
Belgium has sought to root out people suspected of militant links over the past seven years. A Brussels-based Daesh cell was involved in attacks on Paris in 2015 that killed 130 people and on Brussels in 2016 in which 32 died.
Ramming vehicles into crowds has become more common as a weapon used by militants in Europe and white supremacists in the United States because such attacks are inexpensive, easy to organise and hard for authorities to prevent, experts say.
