PARIS - Police gradually began reopening Paris' Gare du Nord train station early on Tuesday morning (May 9) after earlier evacuating the platforms and cordoning off the area because of a security alert, Reuters reported citing local police authorities.

A Reuters witness at the scene said there were between 20 to 30 police vans outside the station in central Paris with security forces wearing balaclavas and carrying assault weapons.

"End of security checks. Gradual return to normal," Paris police said in a tweet.

Authorities gave no other details about the nature of the operation, but earlier media reports said the police were looking for three "dangerous" terror suspects as part of the operation.

Social media was rife with images of a large police presence at the station, with French police tweeting to confirm that checks were being carried out.

According to the Le Parisien newspaper, the focus of their search was a high speed train from Valenciennes in northern France.

Some 200 people were on board the train.

The paper added that the names of the three suspects, all men, had been handed to French security services by a foreign country, and that the trio had been spotted in Bordeaux and Marseille, as well as in Paris.

Security has been stepped up across France because of Sunday's presidential runoff, which Emmanuel Macron won.

Over the past two years, the country has been hit by a series of Islamist militant attacks in Paris and other cities in which more than 230 people have been killed.