The discovery of a huge unexploded Second World War-era bomb near the tracks severed Paris's high-speed rail links with London and Brussels on Friday, dashing weekend getaway plans and causing cascading travel disruptions.

Eurostar, operator of sleek high-speed trains between the U.K. and the continent, announced the cancellation of all its services to and from Gare du Nord in Paris, which serves international destinations such as the EU capital of Brussels, and the Netherlands, as well as the main Paris airport and many regional French destinations.

French national rail operator SNCF says the Gare du Nord habitually sees 700,000 travellers per day, but on Friday commuters were greeted with bright red signs warning of disruptions.

"All Eurostar trains are cancelled to and from Paris today," the rail operator said in a statement sent to The Associated Press. "Eurostar sincerely apologizes for the disruption and understands the inconvenience this may cause. We appreciate everyone's patience and co-operation during this time."

French Transport Minister Philippe Tabarot said the huge disruptions were caused by the discovery of a bomb that weighed half a tonne. Workers found it overnight while doing earth-moving near the tracks in the Seine-Saint-Denis region that borders Paris to the north. 

Bomb disposal experts were called. Tabarot said a "a quite large" security perimeter was set up around the bomb-disposal operation and people were evacuated. He urged commuters to postpone rail trips.

Bombs left over from the First and Second World Wars are regularly discovered around France, but it is very rare to find them in such a people-packed location.

At London's St Pancras station, Eurostar's London hub, travellers scrambled for alternatives. Fridays are invariably busy there with thousands of people leaving and arriving for weekend breaks. Passengers were advised to try taking trains to Lille in northern France or to fly to Paris. 

"We're looking up flights, but our options are limited," said Lauren Romeo-Smith, part of a group that had a birthday weekend in Paris planned.