Some 800,000 passengers were affected by the attacks on France's high-speed train network
Some 800,000 passengers were affected by the attacks on France's high-speed train network. AFP

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.

The 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.

Around 800,000 passengers are expected to be affected over the weekend as the damage is heavy and labour-intensive to repair.

"Early this morning, coordinated and prepared acts of sabotage were perpetrated against installations of SNCF," the national rail operator, Prime Minister Gabriel Attal said.

"There are huge and serious consequences for the rail network," he added, while security services are hunting the culprits.

SNCF chief executive Jean-Pierre Farandou said that the attackers had started fires in "conduits carrying multiple (fibre-optic) cables" that carry "safety information for drivers" or control the motors for points.

"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," he added.

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.

CEO Farandou said that railway workers doing night maintenance in central France spotted unauthorised people, who then fled when the workers called in police.

Multiple services between Paris and London via northern France were also cancelled, the Eurostar company said, with others suffering delays as they divert onto lines not meant for high-speed trains.

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.

The RATP has laid on a denser schedule throughout the day to bring spectators to and from the opening ceremony.

France's intelligence services were scrambling to determine the perpetrators of the sabotage, a security source told AFP.

The source added that the arson method used resembled past attacks by extreme-left actors.

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.

The attacks happened hours before the Olympics parade on Friday evening that will see up to 7,500 competitors travel down a six-kilometre (four-mile) stretch of the river Seine on a flotilla of 85 boats.

It will be the first time a Summer Olympics has opened outside the main athletics stadium, a decision fraught with danger at a time when France is on its highest alert for terror attacks.

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.

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.

Graphic designer Katherine Abby, 30, clung to hope that her trip would only be delay and not cancelled. She booked her tickets for Biarritz, a popular southwest beach resort, weeks ago.

"It's my only vacation of the year," said Abby, who was travelling with her husband.

"I've been waiting for this moment for a year, I would be pretty demoralised to have to cancel this trip, especially when you see what Paris looks like with the Olympic Games," she said.

"We're pretty upset, it's a bad first impression" of France, said Ellie Scott, 24, an Irish tourist in Bordeaux hoping to reach Paris for the Olympics.

She and her sister Maya, 21, planned to refund their tickets and rent a car instead for a six-hour drive to the capital.