Meanwhile, three American men are being hailed as heroes for overpowering a heavily-armed gunman.
The gunman's weapons were said to include a Kalashnikov, a knife, an automatic pistol and cartridges. Two of the American men who overpowered the gunman, Spencer Stone and Alek Skarlatos, are members of the Air Force and the National Guard respectively. The suspect is known to the French security services.
The man detained is a 26-year-old Moroccan. He is being questioned French anti-terrorist officers have taken over the case.
The gunman's weapons were said to include a Kalashnikov, a knife, an automatic pistol and cartridges. The Belgian Federal Judicial authorities have also opened an investigation.
Meanwhile police in the northern French city of Arras are carrying out a fingertip search of the train.
“Just normal passengers”
Two of the American men who overpowered the gunman, Spencer Stone and Alek Skarlatos, are members of the Air Force and the National Guard respectively.
Their friend, Anthony Sadler, was travelling with them and described the scene.
Mr Sadler told journalists: "As he was cocking it to shoot it, Alek just yells, `Spencer, go!' And Spencer runs down the aisle."
"Spencer makes first contact, he tackles the guy, Alek wrestles the gun away from him, and the gunman pulls out a box cutter and slices Spencer a few times. And the three of us beat him until he was unconscious."
Chris Norman, a British man living in France, was also hurt while trying to subdue the attacker.
He told BBC radio "I came in at the end of it all and helped get him under control," he said at a news conference in Arras.
"The guy pulled out a cutter and started cutting Spencer - he cut behind his neck and nearly cut his thumb off."
Also speaking on BBC radio Christina Coons from New York said she ducked under her seat when she heard shots.
She said: "None of these men were in uniform. They were just regular passengers, this afternoon, who stepped up to the plate as soon as they saw what was happening."
Michel: “A terrorist attack”
In a statement issued on Friday evening, the Belgian Prime Minister Charles Michel (Francophone liberal) described Friday evening’s on the Amsterdam to Paris train as “terrorist attack”.
Mr Michel met with the Interior Minister Jan Jambon (Flemish nationalist), the Chief Commissioner of the Federal Police Service and the security services on Friday evening. It was decided that security will be step up.
However, neither Mr Michel, nor Mr Jambon have elaborated on what this will mean in practice.
However, VRT radio’s legal editor Liesbeth Indeherberge says that this probably means that security on international trains will be reviewed.
Our colleague adds that "Everyone is extremely cautious. The CCRV images from Brussels South will be analysed. Sources within the security services say that that it is too early to confirm whether the gunman is also known to the Belgian security services.