An attacker carrying machine gun wounded three people on Friday on a high-speed train in France before being brought down by passengers.
An American soldier, French actor Jean Hugues Anglade and a Briton were injured in the attack.
French Interior Minister Bernard Cazeneuve said the French anti-terrorism prosecutor was investigating the incident.
“As always where an act that could be terrorist in nature is involved, the greatest care and the greatest precision will be used.”
The train was going to Paris from Amsterdam. It is said the attacker allegedly got on the train in Brussels. The attacker was arrested where the train stopped at Arras station in northern France.
French media said the gunman was known to the intelligence services. The suspect has so far insisted on not talking to police in Arras. Belgian Prime Minister Charles Michel said the incident was a "terrorist attack."
A spokesman for French railway SNCF said on iTele television, “The man was armed with automatic weapons and blades. He was stopped by passengers.”
Anthony Sadler, Aleck Sharlatos and Chris Norman living in France, three men who helped to disarm the attacker on a train pose with medals they received for their bravery at a restaurant in Arras.
US President Barack Obama praised Americans on for the "courage and quick thinking" of the passengers.
“The President expressed his profound gratitude for the courage and quick thinking of several passengers, including U.S. service members, who selflessly subdued the attacker,” the White House said in a statement.
“It is clear that their heroic actions may have prevented a far worse tragedy.”
Police officials said the armed man was 26 years old and of Moroccan origin. The attacker’s motives were not known.
French President Francois Hollande said they were cooperating in the investigation with Belgium.
The Belgian government will take strict security measures, a spokesman said.
Since Charlie Hebdo attack which killed 17 people in Paris in January, France has taken high security measures. In July, French officials said they had arrested four people who attempted an attack on a senior French military official.