Belgian security forces have carried out multiple raids across the country, arresting 12 suspects believed to be planning attacks.
Flemish public broadcaster VTM said on Saturday that the people arrested overnight were suspected of planning an attack in Brussels this weekend during one of Belgium's matches in the Euro 2016 football tournament.
The federal prosecutor's office said in a statement that police targeted about 40 locations in a case that needed "immediate intervention".
"In connection with a criminal investigation concerning terrorism... 40 persons were taken for questioning. Twelve among them were arrested. The investigating judge will decide on their possible detention later today," the statement say.
The raids took place in 16 communes in Brussels, Flanders and Wallonia and "passed off without incident," the statement said.
The federal prosecutor said "no arms or ammunition have been found up to now".
Europe is on high alert as the Euro 2016 tournament is under way in France.
Al Jazeera's Nadim Baba, reporting from Paris, said: "People in Belgium and France are extremely aware that there is an ongoing threat.
"The French police have been very stretched, dealing not only with security around match venues but also with hooliganism."
France has deployed 90,000 security forces for the tournament, but French President Francois Hollande has said that the threat of attacks will not stop the event from being successful.
In March, Brussels' Zaventem airport and the Maelbeek metro station were attacked, leaving 31 people dead, including the bombers, and wounding at least 270 in the worst such incident in Belgian history.
The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group said it was responsible for the bombings.
The group also claimed responsibility for coordinated attacks in Paris in November which killed 130 people.