“It is an accident, a tragic one. The car hit the group and tried to carry on but it was quickly stopped by the police,” police spokeswoman Cristina Ianoco said.
“The driver and the other people in the car have been detained,” she added.
Police denied earlier reports in some Belgian media that the crash followed a high-speed police chase.
“Deepest condolences to the families and friends of those killed and injured in the incident this morning,” Belgian Interior Minister Annelies Verlinden wrote on Twitter.
“What was supposed to be a great party turned into a tragedy,” she added. “We are monitoring the situation closely.”