Knives and other weapons were found in the car of a man detained earlier who had driven dangerously through a crowded area in Belgium, authorities say.
Belgium's federal prosecutor said the individual was arrested on Thursday after the vehicle tried entering a shopping area in Antwerp at high speed.
A number of suspicious objects were later found inside the vehicle.
The federal prosecutor reported that the man was a French national living in France.
The mother of a French schoolboy injured in the Westminster terror attack has told how she and her husband believed their son had died.
The teenager, known only as Thomas, was one of three pupils hurt as they walked along Westminster Bridge during a school trip to London.
Thomas suffered a head injury and fractures to his legs, his mother - who has not been named - told French media.
The students, aged between 15 and 16, were pupils from Lycee Saint-Joseph de Concarneau, Brittany.
The teenager's mother said: "It was the school who told us yesterday afternoon that the attack had taken place and our son was one of the victims.
"At that time, we had no idea about his health. My husband and I thought he had died. After an hour, we were told he was injured. It felt extremely long."
Thomas' parents were rushed to London on Wednesday evening to see their son, who she said was "relieved and happy to see us".
She said the pupil had little memory of what happened and they had to explain to him "the attack, the runaway car on the bridge".
English Football League clubs will hold a minute's silence ahead of games this weekend as a mark of respect to those who have lost their lives or were injured in yesterday's terror attack in Westminster.
Former US president Barack Obama has taken to Twitter to express his condolences to the victims of Wednesday's terror attack.
But Mr Obama, who led the US for two terms, said that nothing could "shake the strength and resilience" of Britain.
The Duchess of Cambridge, speaking at an event in London, has sent her "thoughts and prayers" to all those "sadly affected by yesterday's terrible attack in Westminster".
Speaking as the Out Of The Blue educational films - which promote the psychological well-being of parents and their children - were released at an event in central London, the Duchess said: "Before I begin, I know you would all want to join me in sending our thoughts and prayers to all those sadly affected by yesterday's terrible attack in Westminster.
"We will be thinking of all the families, as we discuss the important issues we're here to talk about."