It was not immediately clear who was behind the suspected attacks, and authorities announced no arrests. The Public Prosecutor’s Office opened an organized crime investigation rather than a counterterrorism investigation, indicating that a range of individuals or groups could be responsible.

The agency is investigating charges including “damage to property affecting the fundamental interests of the nation” and “attacks against an automated data processing system.” Each of these potential charges could carry a lengthy prison sentence.

Jean Castex, the head of Paris’s transport system and a former French prime minister, ordered an “increased level of vigilance” across the capital region’s network. “We are deploying in all areas,” he said, according to the French public broadcaster, with a particular focus “on the sensitive locations of our networks.”

No one was reported injured, but French officials said the impact on travelers and the country’s reputation ahead of the Olympics was severe. “The consequences for the rail network are enormous and serious,” Attal said in a message on X, adding that “our intelligence services and law enforcement are mobilized to find and punish the perpetrators of these criminal acts.”

Security has been a major priority in the run-up to the Paris Games. Most of the preparations have been kept secret, but plans have been announced for 15,000 soldiers, 35,000 police officers and 22,000 private contractors to work in and around stadiums and sports facilities across the country.

The biggest security concern was expected to be a terrorist attack, like in Paris in 2015 or Nice in 2016, but there are also concerns about cyber attacks. The French cybersecurity agency was quick to say on Friday that the train disruptions had no cyber element.

Concerns have been heightened by reports that Russia is paying amateur saboteurs to carry out operations across Europe. On Tuesday, French police arrested a Russian national in connection with a suspected plot to cause “destabilization” during the Games, prosecutors said.

Tensions in the Middle East have also been a major factor in security concerns at the Games, with armed guards escorting Israeli athletes to their first soccer match against Mali on Wednesday amid protests over the war in the Gaza Strip. There have also been online threats against the Israeli team, evoking memories of the terrorist attack that killed Israeli athletes at the 1972 Munich Games.

In France and other European countries, railway arson has occurred before, usually attributed to criminals and sometimes to political activists. However, the scale of this incident and its timing during the Olympics casts these recent incidents in a new light.

The French rail network was in some ways an obvious target ahead of the opening ceremony. Weeks before the Olympics, tickets to and from Paris were not on sale for this Friday, with SNCF citing a lack of clarity over security measures.

The suspected sabotage targeted three French high-speed lines, just as 250,000 people were due to board the trains on Friday and 800,000 over the weekend.

So far, it appears to have had less effect on the Olympics than on the Parisians who sought to flee their city to celebrate the sacred national tradition of the August holiday.

The last weekend of July is usually one of the busiest travel periods in France, as Parisians go on holiday. Unlike in normal years, many travellers were also expected to head to the capital to attend the opening ceremonies or Olympic games in the coming days.

“We have damage and deliberate fires in lines where a lot of cables run, cables that are used for signaling,” Jean-Pierre Farandou, the head of the SNCF, told reporters. “They have to be repaired, cable by cable. It is very delicate work.”

Before train services can resume, Farandou said, all the repaired signals will also have to be tested. “That takes time,” he said.

The Eurostar train, one of the UK’s main transport routes to the European continent, also announced delays “due to coordinated acts of vandalism in France, which affected the high-speed line between Paris and Lille” and said trains were arriving in France an hour and a half later than scheduled. It advised travellers to postpone their journeys and said a quarter of its train routes had been cancelled through the weekend.

“At the moment, it is probably one or more acts of vandalism, a kind of coordinated sabotage,” French Sports Minister Amélie Oudéa-Castéra told Le Parisien. “We will assess the impact for today, for travellers, the impact for the athletes and for this weekend.”

The SNCF said its teams were on site to identify the problem and begin repairs. “Some trains are being diverted, a large number of trains have been cancelled,” the organisation said.

Attal thanked “the firefighters who intervened at the affected locations” and “the SNCF agents who will carry out the necessary work to restore the network” – and also the affected travelers for “their patience, their understanding and the civic spirit they are showing.”

Eloise Lecat sat on the floor of Gare Montparnasse, the Paris train station hardest hit by the disruption, with her enormous suitcase and her 3-year-old daughter Léonie on the floor in front of her.

They were headed to Pays Basque to visit Lecat’s mother for the August vacation — which coincides with this year’s Olympic Games. But their 10:15 a.m. train was canceled because of the overnight disruptions, so Lecat frantically searched for other transportation options — bus, car or something else to get them out of Paris. Léonie wanted to see her cousins, Lecat said. “We’ll look for a good solution and see what happens.”

Bisset reported from London. Claire Parker in Paris and Paul Schemm in London contributed to this report