Police gradually began reopening Paris' Gare du Nord train station early on Tuesday morning after earlier evacuating the platforms and cordoning off the area because of a security alert, local police authorities said.

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.

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 militant attacks in Paris and other cities in which more than 230 people have been killed.