A gunman has opened fire on passengers on board a train travelling from Amsterdam to Paris, injuring three people.
Two of the wounded are in a critical condition.
The Foreign and Commonwealth Office said it did not believe British nationals were among the injured.
French interior ministry spokesman Pierre-Henri Brandet confirmed that the suspect was arrested after the high-speed Thalys train stopped in Arras, northern France.
Investigators from France's anti-terror police are leading the inquiry, a spokeswoman for the Paris prosecutor's office said.
Sliman Hamzi, an official with police union Alliance, said the suspect is a 26-year-old Moroccan.
He was armed with an automatic rifle and a knife, according to Christophe Piednoel, a spokesman for national railway operator SNCF.
French actor Jean-Hugues Anglade was reportedly lightly wounded when he broke glass to sound the alarm.
A Foreign and Commonwealth Office spokesman said: "We are aware of reports that shots were fired on a train in France earlier today.
"The French authorities have responded. There are no reports that British nationals have been injured."