Belgian security forces arrested a man Thursday after he drove into a shopping area at high speed in the port city of Antwerp, officials said.
Authorities found a rifle and bladed weapons in the car after the suspect, identified by prosecutors as 39-year-old Mohamed R., tried to flee and was detained in the northern city.
The man was "under the influence of something" but it was not clear what substance, a source close to the investigation told AFP.
Authorities were not certain if it was an attempted attack and the incident remained under investigation, several Belgian sources added on condition of anonymity.
The Belgian prosecutor's office said the man, identified as 39-year-old Mohamed R., was a French national but a French police source told AFP he was Tunisian legally residing in the French city of Lens near the Belgium border.
"He has an address in Lens and as far as we know at the moment, he is not known for large-scale criminal acts," the source said.
"His only previous convictions are minor, such as drunk driving or drug use. He was not on the (French) list of known extremists and according to the initial findings of the investigation had not been flagged up as being radicalised," the source added.
Pierre Camarre, the man's neighbour, told AFP he was about 1.85 metres (six feet) tall "with no beard and wore Western clothes."
The incident jangled nerves following attacks at the Orly airport in Paris and Wednesday's carnage in London, and coming the day after the first anniversary of the Brussels suicide bombings that killed 32 people.
"A vehicle with French plates has tried to drive at high speed into the Meir (shopping street) so that pedestrians had to jump aside," Antwerp police chief Serge Muyters told a news conference.
"Our army colleagues forced the driver to stop but he pulled away and ran a red traffic light. We sent a special forces team and the car and the driver were stopped," he added.
"A man in camouflage was taken away."
Images on social media showed investigators searching a burgundy-coloured vehicle near the bank of the Scheldt river.
- 'Very high speed' -
The Belgian federal prosecutor's office said the suspect was driving at "very high speed" and that "at different times pedestrians were placed in danger."
"Various arms were found in the boot, bladed weapons, a pump-action rifle and a container of as yet unidentified liquid," the prosecutor said in a statement.
Bomb disposal experts attended the scene.
"In light of what has initially been gathered, and taking into account what happened in London yesterday, it has been decided to send this case to the federal prosecutor," the statement added.
Belgian Prime Minister Charles Michel said security services in the Flemish-speaking city "did an excellent job" and the government was following this "suspicious incident" closely.
A French source close to the investigation said the man had "spent the night in a holding cell in Belgium before being released on Thursday morning."
A woman close to the suspect had called the police on Wednesday evening complaining that he had stolen her bag, the source added.
Meir is the main commercial street in Antwerp's historic centre and is mostly pedestrianised. It is one of the country's biggest shopping areas.
The Antwerp incident put Europe further on edge after the attack on the British parliament that killed three people plus the attacker, and the incident at Paris's Orly airport on Saturday when a man was shot dead after grabbing a soldier's rifle.
With soldiers deployed at key sites, Belgium has been on high alert since March 22 last year when suicide bombers attacked Zaventem airport and Maalbeek metro station, killing 32 people and leaving more than 320 wounded.
Belgium suffered a further shock in August when a machete-wielding man shouting "Allahu akbar" (God is greatest) attacked two policewomen in the industrial town of Charleroi, before being shot dead.