A gunman has injured three people - two critically - after he opened fire on a high-speed French train travelling from Amsterdam to Paris.

The suspect, who was reportedly armed with a rifle, automatic pistol, a knife and nine magazines of ammunition, opened fire on the Thalys train at around 6pm local time (4pm BST), and was apprehended by two American passengers, believed to be US Marines.

The Pentagon confirmed that one US military member received non-life-threatening injuries in the incident, but did not confirm what branch of the military they came from.

The train was travelling through Belgium at the time of the incident, which culminated in the gunman being arrested in the northern French border-town of Arras.

French investigating police in protective clothing collect clues inside the Thalys high-speed train (Image: Reuters) (REUTERS/Pascal Rossignol)

Describing the American passengers as "courageous", French Interior Minister Bernard Cazeneuve said that one of the men was hospitalised with serious wounds. A second person was also seriously hurt and a third is being treated for minor injuries, according to a statement from President Francois Hollande's office.

Of the three injured, one was hurt by a gunshot and the other with a knife, according to the French daily Le Monde.

Most of the train's 554 passengers were taken to a gym hall shortly after the train was stopped to be identified, according to Le Monde.

French investigating police look at rifle magazines on the platform in Arras, France (Reuters) (Pascal Rossignol /Reuters)

The American passengers "were particularly courageous and showed great bravery in very difficult circumstances", said Cazeneuve, adding that "without their sangfroid we could have been confronted with a terrible drama".

The pair were off-duty members of the military, and approached the man after hearing the sound of a heavy weapon being loaded in the toilet, the La Voix du Nord newspaper reported. They responded by stopping him from entering the main train aisle.

Investigators gather by the Thalys train in Arras station (AFP) (PHILIPPE HUGUEN/AFP)

The train was evacuated following the shooting, and the suspect was taken into custody, Pierre-Henri Brandet, a spokesman for the French Interior Ministry has said.

French actor Jean-Hugues Anglade was reportedly lightly wounded when he broke glass to sound the alarm.

French judicial police stand on the train platform near weapon cartridges and a backpack in Arras (Image: Reuters) (REUTERS/Pascal Rossignol)

A Kalashnikov was used during the shooting, according to reports from La Voix du Nord.

Speaking on the BFM TV station, Brandet added that the identity of the gunman and the motivation for the attack is unknown.

However, Sliman Hamzi, an official with the police union Alliance, told the AP news agency that the man was a 26-year-old Moroccan.

A police investigator videos the crime scene inside the train where the incident occurred (AP)

Phillipe Lorthois, also from Alliance, told French TV that the gunman shot one victim and injured another with a bladed weapon.

Officers from France's special anti-terror police are leading the investigation, a spokeswoman for the Paris prosecutor's office said. Following his arrest, he has refused to speak to police in Arras, Le Monde reported.

The suspect has appeared on a French intelligence bureau list, according to the AFP news angecy.

Belgian Prime Minister Charles Michel, writing on Twitter, called the incident a "terrorist attack".



Crime and forensic investigators at Arras station (Image: AFP) (AFP PHOTO / PHILIPPE HUGUENPHILIPPE HUGUEN/AFP/Getty Images)

President Hollande is co-operating with Belgian Prime Minister Charles Michel, according a statement from the French government.

The Foreign and Commonwealth Office said it did not believe British nationals were among the injured, following previous reports that a member of the British military stopped the gunman.

More to follow