Police have arrested seven people to foil a planned terror attack in France, the interior minister has said.

The suspects of French, Moroccan and Afghan origin, aged between 29 and 37 years old, were detained in Marseille and Strasbourg overnight Saturday into Sunday.

Bernard Cazeneuve told a televised news conference on Monday the arrests prevented "a terrorist act that had been envisaged for a long time".

The interior minister said six of the suspects were not known to the intelligence services.

He did not reveal the intended targets of the plot, but did say "the foiled attack was a coordinated attack aiming to target several sites simultaneously".

The weekend raids followed an eight-month investigation by security services.

"Credible information made these arrests necessary," an unnamed security source told the AFP news agency.

Mr Cazeneuve linked the arrests to raids carried out by French police shortly before the start of the Euro 2016 football championship hosted by France in the summer.

The announcement comes five days before the opening of the famed Christmas market in Strasbourg - which was the target of a failed extremist plot in 2000.

The city has been on high alert ahead of the opening, with local officials warning it could be postponed or cancelled in the event of serious threats.

A total of 418 people have been arrested in France for suspected links to terror networks since the start of the year, including 43 people this month so far, in anti-terror operations.

The country remains in a state of emergency a year after coordinated attacks by Islamic State jihadists in Paris killed 130 people.