The unexploded bomb was found "in the middle of the tracks" about 2.5 kilometres (1.5 miles) from the station overnight during maintenance work carried out in the northern Paris suburb of Saint Denis, the national SNCF rail company said.
The bomb "dates back to World War II," the RER B suburban train, whose trains from Gare du Nord heading towards Charles de Gaulle airport have also been suspended, wrote in a post on X.
The Gare du Nord station lies in the north of the French capital and is the country's busiest train hub, hosting the Eurostar trains in addition to regional and suburban service, according to the SNCF.
Many passengers could not mask their irritation at the sudden change of plans.
"I've been waiting since 6am for an RER to Goussainville," said 55-year-old Corinne Schiavenato, referring to a town north of Paris.
"I tried to take a replacement bus but it's impossible, too many passengers, they're packed. I'm self-employed, I have a client who has been waiting for me since 7:00 am."
Others took the disruptions in stride. "It's funny, I've just come back from Lorient where unexploded shells are discovered all the time," said Chloe Ternand, referring to a town in western France.
Transport minister Philippe Tabarot warned that rail traffic at Gare du Nord would remain "heavily disrupted all day" on Friday, which usually sees an increase in commuters due to weekend travel.
Speaking to broadcaster Sud Radio, Tabarot called on travellers to "avoid going to the station" and postpone their trips, expressing hope that traffic could resume Friday afternoon.
Eurostar urged passengers to change their journey "for a different date." Eurostar cancelled all its trains leaving from or heading to Paris.
"Traffic is completely halted to and from Gare du Nord. As a result, all Eurostar trains are cancelled to and from Paris today", affecting the routes to London and Brussels, the cross-Channel operator said.
Its services from London to Brussels and London to Amsterdam, which do not go via Paris, were running normally, it added.
In London, crowds gathered at St Pancras railway station after Eurostar trains were cancelled.
Passengers were being advised to go to the northern French city of Lille or travel by plane.
"We would have thought there would be more Eurostar staff, to be honest," said Lauren Romeo-Smith, part of a group on their way to a birthday weekend.
"We're looking up flights, but our options are limited."
This was not the first time that the discovery of a wartime bomb paralysed rail traffic in the French capital.
In 2019, the discovery of such a bomb interrupted traffic between the Saint-Lazare station and the western suburbs of Paris.
Unexploded ordnance is still found across Europe, particularly in Germany where the ground remains riddled with it and bombs are regularly discovered on construction sites, 80 years after World War II.
In August 2023, around 13,000 people were told to temporarily leave their homes in the western German city of Duesseldorf after a one-ton United States-made bomb was found.
Gare du Nord is one of the busiest train stations in Europe, carrying more than 226 million passengers in 2023, according to SNCF.
(Agencies)