Traffic ground to a halt at one of France’s busiest train stations on Friday after an unexploded World War II bomb weighing more than 1,000 pounds was uncovered just north of Paris, causing travel chaos that extended across the English Channel.

The bomb was discovered in the Saint-Denis suburb during overnight work on tracks that lead into the Gare du Nord, a major Parisian transit hub that serves northern France and other parts of Europe, including Britain.

“Traffic has been totally interrupted since this morning to and from Gare du Nord” at the request of the Paris police, France’s national railway company S.N.C.F. said in a statement, adding that trains would start running again once the police completed demining operations.

The bomb was discovered around 3:30 a.m. about a mile and a half from the Gare du Nord, the company said. Workers were landscaping at a bridge renovation site when an earth-moving machine revealed the bomb, which had been buried about six and a half feet underground.

It is about three feet long and includes more than 400 pounds of explosive material, the company said, adding in a travel notice that “extensive earthworks” were necessary to safely defuse it.

Mine-sweeping crews established a large security perimeter, which included sections of a nearby road and a highway that the police closed off.

Traffic on high-speed and commuter railways was stopped for hours. All Eurostar trains that connect Paris to London, Brussels and Amsterdam were canceled, stranding travelers at the Gare du Nord, which served more than 226 million travelers in 2023.

Across the English Channel at London St. Pancras International Station, crowds of bewildered Eurostar passengers were met with a closed barrier. One of them, Katie Jones, was planning to celebrate her 40th birthday at Disneyland Paris. Her plans for an earlier trip to Paris in 2020 had fallen victim to the Covid-19 pandemic that sent much of the world into lockdown.

“I’m scared of flying, and that’s why we chose the Eurostar,” said Ms. Jones, who sobbed against a pillar as she called friends to relay the news. “The irony.”

It is not uncommon in Europe for construction workers to come across unexploded ordnance from the conflicts that tore the continent apart during the 20th century, and the French railway company said that the area in which the latest bomb was found was “well known for its World War II remains.”

Last month, more than 175 practice bombs that had been used for training in the Second World War were found under a children’s playground in northern England. In 2018, thousands of people were evacuated in Berlin after an 1,100-pound World War II-era bomb was uncovered.

“It doesn’t happen every day, but it happens,” said Fabien Villedieu, a leader of Sud-Rail, a union of national railway workers, who shared a picture of the bulky, cylindrical, rock-encrusted bomb on X.

Mr. Villedieu, who said he had received the picture through an internal information-sharing system for railway workers, noted that the unexploded device was found about 600 feet from a busy express road that circles Paris.

“It’s in the middle of an urban zone,” he said.

The local authorities in Saint-Denis said in a statement that nearby houses with windows overlooking the area had been ordered to evacuate during the demining process, which was expected to last about an hour. Six schools and a retirement home were also put on lockdown as a precaution.

Philippe Tabarot, France’s transportation minister, told reporters in Paris that the authorities were taking all the necessary steps to safely remove the bomb and that train traffic would resume as soon as possible.

That was little solace to the passengers at St. Pancras, whose travel plans were turned upside down by a long-forgotten implement of war.

Some took it in stride. Clara Thompson, 52, who is scheduled to run the Paris Half Marathon on Sunday, quickly booked a flight for Friday evening amid the chaos. “It’s an inconvenience, but I get it,” she said. “They found a bomb and they can’t do anything about it.”

But Allison and Jon Heine, who had just arrived on a red-eye flight from Houston, suddenly had three inconsolable children — ages 7, 9 and 13 — on their hands. Their vacation in Paris had been months in the planning.

“It’s not looking good,” Ms. Heine said. “There aren’t any flights. We heard there might be a bus, but it’s eight hours.”

“They’re super disappointed,” she said, pointing to her children. “Tons of tears.”