Updated 7 pm

France's high-speed rail network was hit by arson attacks that disrupted travel for hundreds of thousands of passengers on Friday, just hours before the opening ceremony of the Paris Olympics.

Rail operator SNCF said the country's high-speed network had been targeted by "malicious acts" aimed at paralysing the system.

Several Eurostar trains between Paris and London were cancelled after the massive sabotage attack, the company said.

"All trains departing and towards Paris will be diverted onto the normal line on Friday," Eurostar said on its website, meaning they will take an extra 90 minutes on a route that usually takes around two hours and 20 minutes.

French security forces are hunting people behind Friday's arson attacks that hobbled the country's high-speed rail network hours before the Olympic Games opening ceremony, Prime Minister Gabriel Attal said.

"Our intelligence services and law enforcement are mobilised to find and punish the perpetrators of these criminal acts," Attal posted on X, calling the attacks "prepared and coordinated acts of sabotage against (rail operator) SNCF's installations" with "huge and serious consequences for the rail network".

Airport at Franco-Swiss border evacuated

An airport located at the Swiss-French border was meanwhile evacuated on Friday for security reasons, hours after the arson attacks on the rail network.

"For safety reasons, the terminal had to be evacuated and is currently closed," the Basel-Mulhouse EuroAirport said on its website.

French sports minister blasts sabotage

The "sabotage" was an attack on "the athletes' Games", Sports Minister Amelie Oudea-Castera said Friday.

"These Games are for the athletes who have been dreaming of them for years and fighting for the holy grail of standing on the podium - and someone's sabotaging that for them," Oudea-Castera told broadcaster BFMTV, adding the Olympics "have been prepared for so carefully by hundreds of thousands of our fellow citizens for almost 10 years".

Suspicion will fall on Russia, whose team is banned from Paris in response to the Kremlin's invasion of Ukraine. France is seen as a particular target due to its support for Kyiv.

However, far-left groups within France have a history of targeting the rail network with arson attacks.

The Paris Olympics are set to open in a spectacular and unprecedented ceremony on the river Seine on Friday with up to 7,500 competitors travel down a six-kilometre (four-mile) stretch of the Seine on a flotilla of 85 boats.

What was targeted?

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 relay "safety information for drivers" or control the motors for points that change rails.

Who is affected?

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.

Of four trains supposed to bring athletes to Paris, two made it, one was delayed and the fourth group was to be shifted to another train, SNCF said.

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 after rail maintenance workers surprised unauthorised people on site.

High-speed rail operator Eurostar said around one in four services across its network linking France, the UK, Belgium, the Netherlands and Germany would be cancelled, including trains from Paris to London.

Others would be slower as they will run on regular lines not designed for high-speed trains.

What happens now?

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".

But by early afternoon Friday, Transport Minister Patrice Vergriete said that some departures were going ahead, with "one in three" trains operating from Paris's Montparnasse station, the terminus of the line towards Bordeaux and the Atlantic coast.

He too urged people not to come to the station without confirmation of their trip.

At Montparnasse, passengers were waiting for information, with display boards showing delays of more than two hours.

"Normal traffic is expected to resume on Monday, July 29," read one of the signs in the departure hall.

Long-distance ride-sharing app Blablacar said it had seen a 150-per cent increase in reservations for Friday compared with expectations.

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. 

Who are the culprits?

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

Minister Vergriete said that the people had been spotted with "vans", and "incendiary devices were found on the scene" of the attacks, calling it an "obscene criminal attack".

"This operation was prepared, coordinated, sensitive sites were targeted, which demonstrates some kind of familiarity with the network to know where to strike," Prime Minister Attal said.

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.

What legal action is being taken?

Paris prosecutor Laure Beccuau said in a statement her office had opened a probe into a suspected bid to undermine "fundamental national interests".

The investigation will also look at suspected damage inflicted by an organised gang and attacks on an automated data processing system.

Her statement described the acts of sabotage as "deliberate damage caused to sites of SNCF on the night of July 25-26, 2024."