Police poured into Gare du Nord station in Paris, evacuating passengers from platforms and sealing off the area in a new security scare to hit the French capital.
Paris police spokeswoman Johanna Primevert told The Associated Press the operation was aimed at "removing doubt" and ended with no arrests, but would not say what prompted it.
A station employee said the operation late on Monday focused on a train from Valenciennes in northern France.
The busy station is the terminus of a large suburban and national rail network as well as Eurostar trains from London.
France is under a state of emergency imposed after a series of Islamic extremist attacks.
The operation came the day after a tense and closely watched presidential election that was won by independent Emmanuel Macron. He has said one of his top priorities will be ensuring France's security.
Police from the special BRI intervention force started pouring into the station just after 11pm local time and asked where the train from Valenciennes was, according to a station employee.
Station staff directed officers to platform 10, where they were quickly joined by more and more police. Within minutes, authorities began evacuating the entire station, the employee said.
By 1.30am, the operation appeared to be winding down, with police on the outskirts of the cordon around the station beginning to pack up and the cordon was gradually pared back.
Dave James Phillips, a 39-year-old technology consultant from Britain, said he was getting off the last train in from London when he saw officers flooding the station.
"There were police immediately, and a chap with his hand on his gun," Mr Phillips told AP. "As we were walking down the platform, one came down and said: 'Rapide, rapide. Out, out.'"
Mr Phillips said police kept pouring into the station as passengers were hustled out and police buses and unmarked cars were "driving quite dramatically up the road".
He said he had since left the area.
"Hopefully nothing happened," he said.
A Eurostar spokeswoman said the incident did not appear to have any relation to its trains or affect its passengers.
The operation began after the last Eurostar arrivals and departures late on Monday.