An anti-terrorism inquiry was launched in France on Friday, after an armed man injured three people, two of them seriously, on a train before being overpowered by two American passengers.
The incident took place at around 17:45 CET (15:45 GMT) on Friday on the high-speed Thalys service between Amsterdam and Paris.
The suspect, a 26-year-old Moroccan, was arrested at Arras station in northern France.
He is said to have been carrying a Kalashnikov, an automatic pistol and a knife.
One of the people seriously hurt suffered a gunshot wound and the other a knife wound.
French Interior Minister Bernard Cazeneuve, who travelled to Arras in the wake of the attack, praised the “great bravery” of the Americans who overpowered the armed man.
“Without their composure we could have been confronted with a terrible incident,” Cazeneuve said.
French media reported that the Americans were US Marines.
French actor Jean-Hugues Anglade, who was also on the train, suffered light injuries when breaking glass to sound the alarm.