An armed man has injured three people on a train between Amsterdam and Paris before being overpowered by two American passengers, officials say.
The incident happened on the high-speed Thalys service near Arras, and the attacker was arrested at Arras station.
Two of the victims, said to be an American and a Briton, were seriously injured, reports said.
The man arrested was a 26-year-old Moroccan. Anti-terrorist officers have taken over the case.
The weapons were said to include a Kalashnikov, a knife, an automatic pistol and cartridges.
One of the two people seriously hurt had a gunshot wound, the other a knife wound.
French media said the arrested man was known to the intelligence services. The suspect has so far refused to talk to police in Arras.
French Interior Minister Bernard Cazeneuve is heading to Arras in the wake of the attack.
Interior ministry spokesman Pierre Henry Brandet added: “Talking about a terrorist motive would be premature at the moment.”
Actor on train
The attack took place at about 18:00 local time (16:00 GMT).
French actor Jean-Hugues Anglade was also lightly wounded when breaking glass to sound the alarm.
Local official Fabienne Buccio praised the passengers for showing “great composure”.
French media said the men who overpowered the suspect were US Marines.
The UK Foreign Office said it was aware of the reports of shots being fired and that was in touch with French authorities, urgently seeking information.
Images shared on social media appeared to show a man being restrained on the station platform in Arras.
“The situation is under control, the travellers are safe. The train stopped and the emergency services are on site,” the Thalys official Twitter account tweeted.
Thalys said several trains had been delayed in the wake of the attack.
France has been on edge since the attack on the Charlie Hebdo magazine and a Jewish supermarket in Paris in January, which left 17 people dead.
And in June a man said to be inspired by the Islamic State group beheaded his boss and tried to blow up a gas plant in southern France.