How many have died and been injured is currently unclear.
According to De Telegraaf, there is no information that it is a terrorist crime.
Instead, it is likely to be an accident, reports Reuters.
"The investigation into the cause of the incident is in full swing," the police wrote on Twitter.
The barbecue party is said to have been organized by a local skating club.
Pictures in the Dutch media show how tables and chairs have been pushed together, and how the Spanish-registered truck is standing in the middle of the site.
- We were having a barbecue and saw a truck stop at the intersection, then it drove straight through the tent, says an eyewitness to Algemeen Dagblad.
- It was horrible.
Several people were dragged along.
A large number of ambulances are at the scene and several ambulance helicopters have been deployed, the police say.
The driver has been arrested by the police, reports De Telegraaf, which refers to the TV channel NOS.
The text is updated.