No one was injured.
A French police official told The Associated Press that a man carrying a knife walked into the station and was flagged to police, who arrested him immediately. Video online shows heavily armed police surrounding a prone man as travellers hurried past.
The official spoke on condition of anonymity because she was not authorised to speak publicly.
The Gare de Nord is one of the French capital's top transit hubs, serving the city's metro, suburban trains as well as intercity and high-speed trains like the Eurostar from London.