A car ploughed into a crowd of early morning carnival-goers in Belgium on Sunday, killing six people and injuring dozens of others, authorities said, as they ruled out an act of terrorism for now.

The tragedy took place around 5:00 am (0400 GMT) as the carnival of Strepy-Bracquegnies, a district in the former coal-mining town of La Louviere, was getting underway.

"At this stage of the investigation, we know that a vehicle slammed into a group (of carnival goers) and that there are six dead and 26 injured, including 10 people whose life is in danger," prosecutor Damien Verheyen told reporters at the town's city hall.

The main suspects, who were arrested, were born in 1988 and 1990, he said, adding that terrorism was not at this stage considered a motive. 

Verheyen said the two came from La Louviere, a town near the French border in Belgium's rust belt, and are not known to authorities for similar acts.

Belgian towns and villages host many street carnivals around the season of Lent, with the parades in Binche and Aalst the best known internationally. 

Like Binche, the carnival of Strepy-Bracquegnies involves participants dressed up as "Gilles", extravagantly attired comical figures who are "called out" to the parade in the early hours.

La Louvriere mayor Jacques Gobert said there were 150 to 200 people participating in the carnival's pre-dawn prologue, including the "Gilles" in their garb.

When they reached a road, "a car coming from behind at high speed literally pulverised a large number of people," he said at the news conference.

Witnesses described a horrific scene of a car that apparently wilfully drove into the carnival-goers that included children.

Fabrice Collingnon, a Belgian TV presenter, told AFP the procession was joyfully underway when in a "snap of the fingers we went from fun and folklore to absolute horror."

The car seemed to "deliberately enter into the pack," said Collingnon.

Another witness, Theo, told RTBF news he saw a car that moved "very fast and didn't brake".

Belgian Prime Minister Alexander De Croo deplored the "horrible news" on Twitter, saying "a community gathering to celebrate has been hit in the heart."

De Croo and other ministers rushed to the scene along with Belgium's King Philippe who met first responders in the Strepy-Bracquegnies sporting hall, where the morning's carnival procession began.

The RIAAL La Louvriere football team said that one of the victims was a steward at the club and that a match scheduled later on Sunday was cancelled.

"Words fail us," the team wrote on its Facebook page.

In neighbouring France, just a few kilometres from the scene of the tragedy, President Emmanuel Macron "assured the Belgian prime minister of his support and friendship after this tragedy", his office said.

In February 2020, a man rammed his car through a carnival procession in Germany, injuring dozens of bystanders including children.

Germany and other countries at the time had been on high alert for car-ramming attacks since December 2016, when an Islamic State group sympathiser ploughed a truck through a Christmas market leaving 12 dead.

German towns had seen several such attacks since, with most carried out by people who were found to have psychological issues.