Belgian authorities have charged three men with terrorism offences, including attempting to commit murder, following a huge overnight operation.
The three men charged on Saturday were named as Samir C, aged 27; Moustapha B, 40; and Jawad B, 29.
The charges they face include attempting to commit murder through terrorism and participating in a terrorist group.
Nine other people who had also been arrested have been released by the investigating judge, the federal prosecutor's office said.
Searches were conducted overnight in 16 municipalities, mainly around Brussels, with checks on 152 storage units.
Europe is on high security alert, with the Euro 2016 soccer tournament under way in neighbouring France.
PM Charles Michel said security would be stepped up at public events.
He tweeted (in French) after a meeting of the Belgian security council to say events would go ahead as planned, urging the public to remain calm.
Belgian media had earlier reported that militant Islamists may have been planning to attack the football fan zone in Brussels, where Belgium's games tournament in France are screened.
No arms or explosives were found during the searches, which passed off without incident, the federal prosecutor said in an earlier statement.
Forty people were initially taken in for questioning.
A French police couple were stabbed to death outside their home on Tuesday in an attack claimed by Islamic State.
In a video posted on social networks, the attacker linked it to the soccer tournament, saying: "The Euros will be a graveyard."
Among the areas where searches took place was Molenbeek, a Brussels district which has become notorious because of its associations with jihadists.
Officials said the operation had been launched after investigations which "necessitated an immediate intervention".
The country is under a level three terror alert, one below maximum.
There were several warnings of further attacks in recent days.
On Friday evening, four federal ministers including Mr Michel were placed under heightened police protection along with their families, Belgian media reported.
Belgian police were said to have recently received a warning that a group of militants from the Islamic State (IS) militant group had left Syria en route for Europe, planning new attacks in Belgium and France.
An unnamed security source was quoted by Belgian newspaper DH (in French) on Wednesday as saying the group had "left Syria about a week-and-a-half ago aiming to reach Europe via Turkey and Greece by boat without passports".
Meanwhile, a Belgian national named as Youssef EA was arrested on Friday and charged on suspicion of terrorist activities in connection with the March attacks, which were claimed by IS.
On 22 March, 32 people were killed by bombs at a Brussels airport and a metro station, following attacks in Paris in November which left 130 people dead.
Belgian media report that he is the eighth person to have been charged over the attacks, which were carried out by three suicide bombers.
Investigators have established connections between the Brussels bombers and the attacks on Paris.
Several of the Paris bombers came from Belgium and some of the bombs were made in a flat in Brussels.