Dutch police were trying to determine why the truck left the road and careered down the side of a dike in the village of Nieuw-Beijerland just south of Rotterdam.
Police said the crash took place around 6 p.m. local time.
"You can imagine how awful it is, during a neighborhood party a truck drives into this group of people, the shock is enormous," a police spokesperson on the scene told Omroep Rijnmond radio station.
The driver, a 46-year-old Spanish national, was arrested as part of the investigation.
Police said he was not under the influence of alcohol at the time of the crash.
Police are investigating
The rescue mission involved several ambulances and a helicopter, and a number of people were taken to hospital.
"My condolences go out to the victims, their families, eyewitnesses and first responders," Mayor Charlie Aptroot said.
Police worked into the night around the truck before a crane and a tow truck hauled it back onto the road.
The accident happened a day after a van driver plowed into a crowded cafe terrace in central Brussels, wounding six people.
