Arson attacks scrambled France's high-speed rail network for tens of thousands of passengers on Friday, after what officials called premeditated acts of "sabotage" just hours before the Paris Olympics opened.
Friday's attacks were launched as the French capital was under heavy security ahead of the Games opening ceremony, with 300,000 spectators and an audience of VIPs expected at the event.
Fires that affected France's Atlantic, northern and eastern lines led to cancellations and delays at a time of particularly heavy traffic for summer holiday travel.
"Early this morning, coordinated and prepared acts of sabotage were perpetrated against installations of SNCF," the national rail operator, Prime Minister Gabriel Attal said.
SNCF chief executive Jean-Pierre Farandou said the attackers had started fires in "conduits carrying multiple (fibre-optic) cables" that carry "safety information for drivers" or control the motors for points.
Around 800,000 passengers are expected to be affected over the weekend as the damage is heavy and labour-intensive to repair.
France's rail network was expected to be busy this weekend, not only due to the Olympics but also as people return from or leave for their summer holidays.
"There are huge and serious consequences for the rail network," added Attal.
Passenger services chief Christophe Fanichet said there were delays of 90 minutes to two hours on services between Paris and France's north and east.
"We ask people please not to come to the station, because if you haven't heard from us, your train won't be running," Fanichet told reporters.
One major branch of the network, the line to France's southeast, was spared.
Multiple services between Paris and London via northern France were also cancelled, the Eurostar company said, with others suffering delays as they diverted on to lines not meant for high-speed trains.
Farandou of SNCF said: "There's a huge number of bundled cables. We have to repair them one by one, it's a manual operation" requiring" hundreds of workers."
At Paris's Montparnasse train station, passengers were waiting for information, with display boards showing delays of more than two hours.
SNCF said there would be no trains at all from Montparnasse before 1:00 pm (1100 GMT).
"Normal traffic is expected to resume on Monday, July 29," read one of the signs in the departure hall.
Paris's RATP transport network was also operating under "increased vigilance" following the railway attacks, its chief executive Jean Castex said as he visited a control station.
SNCF CEO Farandou said that railway workers doing night maintenance in central France had spotted unauthorised people, who fled when the workers called in police.
France's intelligence services were scrambling to determine the perpetrators of the sabotage, a security source told AFP. The arson method used resembled past attacks by extreme-left actors, the source added.
In September, arson attacks on conduits holding railway cables caused travel chaos in northern Germany, with a claim of responsibility posted to an extreme-left website.
Paris prosecutors opened a probe into a suspected bid to undermine "fundamental national interests," Paris prosecutor Laure Beccuau said in a statement.
Her statement described the acts of sabotage as "deliberate damage caused to sites of SNCF on the night of July 25-26, 2024."
