Belgian police arrested 12 people suspected of planning new attacks after about 40 houses were searched across the country overnight, the federal prosecutor's office said on Saturday.
Europe is on high security alert, with the Euro 2016 soccer tournament underway in neighboring France, after Islamist suicide bombers killed 32 people in Brussels in March following attacks in Paris in November in which 130 people died.
"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 office said in a statement.
"The investigatory results necessitated an immediate intervention. The investigation continues."
Investigators have found links between the Brussels and Paris attackers, some of whom were based in Belgium.
Belgium's federal prosecutor's office said no weapons or explosives were found during the searches, which also involved 152 garage lockups.
A French police couple were stabbed to death outside their home on Monday in an attack claimed by Islamic State. In a video posted on social networks, the Larossi Abballa, the attacker, linked it to the soccer tournament, saying: "The Euros will be a graveyard."
A spokeswoman for Paris public prosecutor Francois Molins said two people close to Abballa would meet a judge on Saturday.
Flemish public broadcaster VTM said the people arrested overnight were suspected of planning an attack in Brussels this weekend during one of Belgium's soccer matches.
Belgian newspaper Le Soir said areas where fans watch matches were potential targets, as well as other crowded areas like shopping centers and stations, citing a security source.
Belgian police on Wednesday received an anti-terror alert warning that a group of Islamic State fighters recently left Syria en route for Europe planning attacks in Belgium and France, security officials said.
Public broadcaster RTBF said the country's crisis center on Friday placed several government ministers, including Prime Minister Charles Michel, under heightened protection.
Police arrested and detained a 30-year-old man named as Youssef E.A, a Belgian national, on Friday on suspicion of taking part in terrorist activities in connection with the Brussels attacks.