Belgian prosecutors said they were charging three suspects with attempted terrorist activity late Saturday, hours after the country's national security council met to review mounting evidence of an "imminent" attack.
A major security sweep was carried out, and Prime Minister Charles Michel said authorities would remain "extremely vigilant, hour by hour." However, there were no major incidents involving Belgium's Euro 2016 football match Saturday, which some had feared might be a target for militants.
The prime minister said "we will take additional ... measures" shortly, but gave no details.
Belgium has remained under a terror alert since attacks in March on the Brussels airport and subway killed 32 people.
The security sweep, which began Friday, rounded up more than 40 people seen as suspected militants, but most were detained only briefly. Twelve detainees were held for further questioning, and nine of those had been released by late Saturday.
Belgian authorities said the three suspects who remained in custody were being charged with attempting to commit terrorist offenses.
A member of France's rapid intervention police force RAID surveys spectators inside Bordeaux stadium, where Belgium was playing Ireland in Euro 2010 football competition, June 18, 2016.
Belgium's Euro 2016 match against Ireland was played in nearby France. The Belgian team won, 3-0.
Viewing parties were held throughout the country, also without major incident, authorities said.
Authorities said a need for "immediate intervention" triggered the two-day security sweep across all parts of the country — in Brussels, Flanders and French-speaking Wallonia. Police in the capital searched homes in 16 municipalities.