Armed police stormed the Gare du Nord railway station in Monday's overnight hours, evacuating passengers and sealing off the area. The operation comes a day after the presidential election won by Emmanuel Macron.
Dozens of police officers and transport staff surrounded the station - the busiest in Europe - in northeast Paris and sealed off all roads leading in.
Paris police said on Twitter they were conducting "checks" at the station, but that the situation was returning to normal.
The operation was connected to a search for three men who are considered to be dangerous, according to a report in "Le Parisien" newspaper. Passengers were unable to exit their trains as armed police entered the station, eyewitnesses said.
The paper reported the men had also been spotted in Bordeaux and Marseilles over the weekend. Officers reportedly searched a train arriving from Valenciennes in northern France, which is thought to have had 200 people on board.
Unconfirmed reports suggested two of the three men being hunted were Belgian nationals while the other was from Afghanistan.
The operation began after the last Eurostar arrivals and departures late Monday. The station is the terminus for many local and international trains, including Eurostar trains from London and Brussels.
Paris police spokeswoman Johanna Primevert told The Associated Press that the operation was aimed at "removing doubt."
A Eurostar spokeswoman said the incident did not appear to have any relation to its trains and did not appear to affect its passengers.
Early Tuesday morning police lifted the evacuation and slowly reopened the station.
France is under a state of emergency imposed after a series of Islamic extremist attacks.
French President-elect Emmanuel Macron has said one of his top priorities will be ensuring France's security.
Earlier in the day riot police clashed with revolutionary groups as thousands of demonstrators gathered at Place de la Republique in the capital calling for 'resistance' to Macron's employment and benefits reform agenda.
jbh/kl (dpa, AP)