Hours before the Olympic Games opening ceremony was due to begin, a coordinated arson attack brought down Paris' rail network.
The train lines link to different cities hosting events over the coming weeks, including Bordeaux and Lille, as well as international stations.
Here's what we know so far:
Where were the fires?
The "deliberate fires" damaged signalling infrastructure at Courtalain in western France, Croisilles in the north and Pagny-Sur-Moselle in the east, national rail operator SNCF said.
These three incidents affected France's Atlantic, northern and eastern lines leading to mass cancellations and delays during the peak summer holiday travel period.
High-speed rail operator Eurostar said about 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 run on regular lines not designed for high-speed trains.
"Early this morning, coordinated and prepared acts of sabotage were perpetrated against installations of SNCF," 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 was affected?
About 800,000 passengers are expected to be affected over the weekend as the damage is heavy and labour-intensive to repair.
About 250,000 were affected on Friday alone, SNCF said.
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.
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.
One branch of the network, the line to France's south-east, was spared after rail maintenance workers surprised unauthorised people on site.
"There are huge and serious consequences for the rail network," Mr Attal said.
Two German athletes in showjumping — Philipp Weishaupt and Christian Kukuk — said they will miss the opening ceremonies because of lengthy delays, German news agency dpa reported.
"It's a real shame, but we would have arrived too late," Weishaupt said.
"There was no longer a chance of making it on time."
According to French paper Le Monde, British Prime Minister Keir Starmer changed his plans to from London to Paris after the Eurostar trains were interrupted.
A spokeswoman said he travelled by plane instead due to the delays and cancellations.
People on X have expressed how the interruptions have impacted them.
One user said she was planning to watch the basketball in Lille on Sunday but the return train had been cancelled, and asked if anyone knew a way back to Paris.
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Another said she had taken the Eurostar to avoid airport chaos, but instead that backfired.
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What do we know about the culprits?
Mr Farandou said railway workers doing night maintenance in central France had spotted unauthorised people, who fled when the workers called in police.
Transport Minister Patrice 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," Mr 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.
Asked about whether they could be linked to Russia, French Sports Minister Amelie Oudea-Castera said "maybe," adding that the attacks could also have been homegrown. She called them "malicious acts".
Other French officials said it was too early to say whether there is any Russian connection.
What happens now?
SNCF said it is now working to repair the network.
"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," Mr Farandou said.
But by early afternoon on Friday, Mr Vergriete said that some departures were going ahead, with "one in three" trains operating from Paris' Montparnasse station, the line towards Bordeaux and the Atlantic coast.
Long-distance ride-sharing app Blablacar said it had seen a 150 per cent increase in reservations for Friday compared with expectations.
SNCF said that at the weekend two out of three high-speed trains linking Paris with western and south-west France would leave as scheduled, while 80 per cent of high-speed trains in northern France would run.
But all travel would see delays.
Eurostar said that one in five of its trains would be cancelled at the weekend and warned of delays on those that do run.
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 suspicions that damage was inflicted by an organised gang and that attacks targeted an automated data processing system.
The investigation is in the hands of criminal prosecutors. Anti-terrorism prosecutors have not been asked to handle the case.
ABC/Wires
