Brussels prosecutors on Sunday said police had carried out 13 new raids in various districts in and around the capital as part of investigations following bomb attacks.
In a statement, the federal prosecutor’s office said a total of nine people were questioned “in the context of a terrorism file”.
Five were later released.
It said it could not give further details.
Police use water cannon as hundreds demonstrate
Police used water cannon to control hundreds of rowdy protesters in central Brussels on Sunday after they ignored an official call for solidarity marches following Tuesday's bomb attacks to be postponed.
Local media said about 450 protesters noisily faced off with white-helmeted riot police in front of the city's colonnaded stock exchange building where wreaths and flowers had been laid out in a tribute to the victims of the attacks.
The crowd, many of whom media described as right-wing nationalists, shouted slogans vehemently denouncing Islamic State which claimed responsibility for the attacks in which 31 people were killed and scores injured.
One banner defiantly read: “United against Islamic State".
Ranks of riot police, carrying shields, hemmed in the crowd and then gradually pushed the protesters back into a street where water cannon trucks opened up.
Prime Minister Charles Michel condemned the demonstrations. "It is highly inappropriate that protesters have disrupted the peaceful reflection at the Bourse (stock exchange). I strongly condemn these disturbances,” he said according to Belga news agency.
The mayor of Brussels, Yvan Mayeur, described the crowd as scoundrels and said they had come into Brussels from other towns, in particular Antwerp.
“I am appalled at what has happened, to see that such scoundrels come here to provoke people at their place of homage,” he said.