An Interior Ministry spokeswoman, Berangere Dubois, said "a high-pressure gas distribution pipeline blew up" early on Friday morning at the Ghislenghien business park, near the town of Ath, which lies close to a major highway.
Firemen and policemen were among the injured, she said.
Earlier reports by Belga news agency, quoting emergency services, said the explosion occurred at around 9:00 am (0700 GMT) at a gasworks which was under construction.
Belgian broadcaster RTBF reported that flames several metres high were shooting from the site and that thick smoke billowed over the scene.
The state-run RTBF radio station said around 60 of the injured, many with massive burns, were taken to an improvised holding centre before being ferried to specialist hospitals by helicopter.