French President Francois Hollande said "everything" would be done to determine the motive of the shooting at Thalys high-speed train in which an assailant injured three men.
"Every effort is being made to shed light on this tragedy and have all the necessary information about what happened," Hollande said in a statement issued by his Elysee Palace office.
Three people were injured, including two in serious condition, the Elysee said. However, local media reports said the injured include one American and a British citizen.
French actor Jean-Hugues Anglade was also injured in the shooting.
The Elysee said the gunman, not identified yet, was arrested in Arras station in France. He was neutralised by two American passengers before being handed over to police.
"Without their courage we would have faced a terrible tragedy," said Bernard Cazeneuve, the interior minister, adding "the greatest care and the greatest precision will be used" in the inquiry launched by a counter-terrorism prosecutor.
Pierre-Henry Brandet, the ministry's spokesman said it is still too early to call it a terrorist attack.
"We have no information neither of the assailant's identity nor of his motivation. For sure, it is a dramatic event and police units and rescue teams are deployed on site," he said.
According to media reports, the suspect is believed to be a 26-year-old Moroccan, who is known to intelligence services.
In a separate statement, French presidency said Hollande and Belgian Prime Minister Charles Michel vowed to step up security cooperation and consolidate efforts to facilitate the investigation into the shooting.
France has been on high alert after January attacks in Paris that killed 17 people.
