Special Belgian police units are searching for a heavily armed soldier with far-right views who they believe poses a serious threat.
The suspect has been named as Jurgen Conings, a military shooting instructor who took weapons from a barracks.
He is said to have made threats in the past against virologist Marc Van Ranst, who led Belgium's public health response to coronavirus.
Mr Van Ranst and his family have been taken to a safe place.
Latest reports say as many as 350 police and some military have been deployed in and around a national park near the Dutch border. Six shots or "detonations" were reportedly heard in the area at 18:35 local time (17:35 BST) but it is unclear if this was gunfire. A motorway going through the area was closed because of the operation.