French officials condemned the attacks as "criminal actions", though they said there was no sign of a direct link to the Games, and prosecutors in Paris opened a national investigation saying the crimes could carry sentences of 15 to 20 years.
"It's a hell of a way to start the Olympics," said Sarah Moseley, a 42-year-old traveller waiting at the Gare du Nord station in Paris as she learned her train to London was delayed.
As Paris authorities geared up for a spectacular parade on and along the Seine River, three fires were reported near the tracks on the high-speed lines of Atlantique, Nord and Est, causing disruptions that affected hundreds of thousands of travellers.
Among them were two German athletes in showjumping who were on a train to Paris to take part in the opening ceremony but had to turn back in Belgium because of the closures.
"There was no longer a chance of making it on time," rider Philipp Weishaupt, who was travelling with teammate Christian Kukuk, told German news agency DPA.
The train disruptions hit Paris's Montparnasse station particularly hard. (AP PHOTO)
There were no known reports of injuries.
The disruptions hit Paris's Montparnasse station particularly hard.
Passengers at St Pancras station in London were warned to expect delays of about an hour to their Eurostar journeys.
French Prime Minister Gabriel Attal said France's intelligence services had been mobilised to find the perpetrators of "acts of sabotage" that he described as "prepared and co-ordinated".
Attal said sabotage and arson that hit key parts of France's high speed rail network on the eve of the Olympics had "a clear objective: blocking the high speed train network".
He said the vandals strategically targeted the axes from the north, east and west towards Paris hours before the capital hosts the Olympics opening ceremony.
Intelligence services scrambled to find who's behind the "acts of sabotage" on the rail network. (AP PHOTO)
It was "a premeditated, calculated, co-ordinated attack" that indicates "a desire to seriously harm" the French people, the rail company's CEO Jean-Pierre Farandou said.
The incidents paralysed high-speed lines linking Paris to the rest of France and to neighbouring countries, Vergriete said, speaking on BFM television.
The attack occurred against a backdrop of global tensions and heightened security measures as the city prepared for the 2024 Olympic Games.
Many travellers were planning to converge on the capital for the opening ceremony, and many holidaymakers were also in transit.
French authorities have foiled several plots to disrupt the Olympics, including arresting a Russian man on suspicion of planning to destabilise the games.
The Paris police prefecture "concentrated its personnel in Parisian train stations" after the "massive attack" that paralysed the TGV high-speed network, Laurent Nunez, the Paris police chief, told France Info television.
Passengers at St Pancras station in London were warned to expect delays on the Eurostar. (AP PHOTO)
Also Friday, the French airport of Basel-Mulhouse on the border with Germany and Switzerland was evacuated in the morning and remained temporarily closed "for safety reasons", the airport said.
It was not clear whether there was a connection to the rail attacks.
Sports Minister Amelie Oudea-Castera said authorities were working to "evaluate the impact on travellers, athletes, and ensure the transport of all delegations to the competition sites" for the Olympics.
Speaking on BFM television, she said: "Playing against the Games is playing against France, against your own camp, against your country."
SNCF said it did not know when traffic would resume and feared that disruptions would continue "at least all weekend".
SNCF advised "all passengers to postpone their journey and not to go to the station", and all tickets were exchangeable and refundable.
Valerie Pecresse, president of the regional council of the greater Paris region, speaking from Montparnasse station, said "250,000 travellers will be affected today on all these lines".
The troubles comes before an opening ceremony has been planned for later Friday, in which 7000 Olympic athletes are due to sail down the Seine past iconic Parisian monuments such as Notre-Dame Cathedral, the Louvre Museum, and the Musee d'Orsay.
