Eleven people were hurt, five seriously, when a van drove into a Christmas market in France.
The driver of the white Peugeot was among those seriously injured in the incident in the main square in the western city of Nantes.
French officials had called for vigilance but warned against jumping to conclusions after a pair of weekend attacks - one in which a driver ran down 13 bystanders in Dijon, and another in which a recent convert to Islam knifed police officers in the central city of Tours.
There are so far no details of the driver nor has any possible motivation emerged.
The driver in Dijon, who had a history of psychological problems, was arrested following the attack in the city in eastern France, while the assailant who stabbed the officers outside the city of Tours was shot to death by police.
None of the victims died.
The prosecutor in Dijon said the driver behind that attack has a long history of severe mental illness and no links to terrorism.
The man, who is 40, has admitted his role in the attack.
He has been in hospital 157 times for psychiatric problems since 2001.
The French-born son of North African immigrants, he acted alone and had no religious motivation, but was upset at the treatment of Chechen children.
He shouted "God is great" to give himself courage to act, and not out of religious belief, it is understood.
The suspect was arrested. Eight people remain in hospital.
Counter-terrorist police are investigating the attack on police in a suburb of Tours on Saturday, which left two officers seriously injured and a third with light injuries following the knife rampage.
The attacker, who was killed by police, was a 20-year-old from Burundi named Bertrand. Police believe he was drawn to radical Islam several years ago by his 19-year-old brother Brice, who has been detained for questioning by police in Burundi.
French Interior minister Bernard Cazeneuve visited Dijon, an indication of the government's heightened concerns days after Islamic extremists renewed calls for individuals to strike in the West.