Vandals prompted a fire at the Gare du Nord train station in Paris that halted traffic to Britain and many other destinations, authorities have said.
The SNCF national rail service said the fire on Tuesday was caused by a "malicious incident", without elaborating.
A police official said the fire started in an electrical station and prompted an immediate cut-off of power to the rails.
The fire was quickly extinguished and presented no danger to passengers, the official said.
No one has yet been detained, but there is no sign of any terrorist intent, said the official.
The incident came amid nationwide jitters following a truck attack in Nice last week that killed 84 people and was claimed by the Islamic State group, the third major attack in France in 18 months.
SNCF told passengers that traffic would resume on commuter lines late on Tuesday night, but passengers on Eurostar trains across the English Channel were told they would have to wait until Wednesday to leave.
Trains to Brussels, Amsterdam and other international destinations also leave from the Gare du Nord.
Theresa Pietrangelo, of Toronto, who was trying to return to London with her daughter Emily, said: "This is a disaster. Nobody knows what's going on and they told us our train isn't leaving tonight.
"We have tickets to fly to Edinburgh tomorrow morning and now I don't know what to do."
They had spent a week in London and this was supposed to be their one fun day in Paris -Tuesday was Emily's 21st birthday.