Hasselt Mayor Hilde Claes said that two more people died overnight. About 140 were injured in the storm, 10 of them seriously, she said.
All the dead were Belgians, Claes said.
Organizers canceled the annual Pukkelpop festival near Hasselt, 50 miles (80 kilometers) east of Brussels. Buses and trains were pressed into service to transfer the 60,000 festivalgoers home.
Thousands of mud-splattered young people, many of them shoeless, trekked down the avenue leading from the festival venue to train and bus stations in Hasselt. Many had stayed on in the camping ground in the vain hope that the performances would continue on Friday.
The brief but violent thunderstorm on Thursday evening tore down concert tents, several trees and main stage scaffolding. Panicked concertgoers ran through fields of mud looking for shelter.
At a joint news conference Friday, Hasselt officials and festival organizers described weather conditions at the event's opening day as exceptional and unprecedented. They said that neither the Belgian weather service nor weather forecasters at a nearby air force base had predicted a storm of this intensity.
Chokri Mahassine, the organizer of the annual festival that was first held in 1985, said he had never seen anything like it.
"I have seen many tropical storms but this was unprecedented," he told journalists. He said he canceled the event "out of respect for the victims, their relatives and friends we felt that the concert could not continue."
"This is the blackest day that any Belgian festival has experienced," Mahassine said. "I would not wish this on anybody."
Belgian Prime Minister Yves Leterme expressed his government's condolences to the families of the victims and said authorities would continue to provide assistance in caring for the injured.
The three-day festival's lineup featured internationally known acts, including Foo Fighters, Eminem and The Offspring.
This was the second deadly incident at an outdoor festival in a week. On Saturday, parts of a stage collapsed at the Indiana State Fair in Indianapolis, killing five people and injuring dozens, when winds of up to 60 miles per hour (96 kilometers per hour) to 70 miles per hour (112 kilometers per hour) hit the site.