Video from the site (link to YouTube) showed stage equipment dangling in high winds near the town of Hasselt, about 50 miles (80 kilometers) east of Brussels, as rain-soaked concertgoers at the Pukkelpop music festival ran for cover .
Organizers estimated some 60,000 people were at the three-day festival, which started Thursday, when the storm broke. Many were leaving the grounds after the storm that turned the festival site into a scene of mud and destruction in a matter of about 10 minutes.
Hugo Simons, Hasselt's head of emergency medical planning, told VRT radio that three people died, 11 were severely injured while 60 sustained light injuries as a result of the storm.
Hilde Claes, Hasselt's mayor, said those injured are taken to several nearby hospitals. Some of those lightly injured were being treated at a local sports complex. More than 20 ambulances were dispatched to the festival ground outside this small east Belgian town.
Chokri Mahassine, the organizer of the Belgian festival, said "we have for now put the festival on hold until we understand the situation completely."
The three-day festival's lineup includes major acts, including Foo Fighters, Eminem and The Offspring.
It is the second deadly incident at an outdoors festival within a week. On Saturday, parts of a stage collapsed at the Indiana State Fair in Indianapolis, killing five people and injuring dozens after winds with a velocity of 60 miles per hour (96 km/h) to 70 mph (112 km/h) hit the site.
Dutch NOS television reporter Rick Hoogkamp, who was attending the concert Thursday, said the sky got dark, winds picked up violently and hailstones pelted those gathered.
He said several tents collapsed. Images of the disaster also showed fallen trees and fallen lighting scaffolds.