High winds and heavy rain struck the Pukkelpop festival near the town of Hasselt in the north of the country, the DH Daily has said.
Video from the site shows stage equipment dangling precariously and soaked revellers running for cover.
According to the Belga news agency, two stages collapsed, one on top of the crowd.
Some giant screens also fell down and trees were uprooted.
It is thought around 60,000 people are attending the event.
Note: The Netherlands has this evening raised the weather alert to ORANGE as the storm is moving over the South of Holland.