PARIS — France’s high-speed rail network was hit Friday with widespread and “criminal” acts of vandalism including arson attacks, paralyzing travel to Paris from across the rest of France and Europe only hours before the grand opening ceremony of the Olympics.

French officials condemned the attacks as “criminal actions,” though they said there was no sign of a direct link to the Games, and prosecutors in Paris opened a national investigation saying the crimes could carry sentences of 15 to 20 years.

As Paris authorities geared up for a spectacular parade on and along the Seine River, three fires were reported near the tracks on the high-speed lines of Atlantique, Nord, and Est, causing disruptions that affected hundreds of thousands of travelers.

Among them were two German athletes in showjumping who were on a train to Paris to take part in the opening ceremony but had to turn back in Belgium because of the closures, and will now miss the ceremony, German news agency dpa reported.

“There was no longer a chance of making it on time,” rider Philipp Weishaupt, who was traveling with teammate Christian Kukuk, told dpa.

The French prime minister, Gabriel Attal, wrote on the social media platform X that France’s intelligence services have been mobilized to find the perpetrators.

Mr. Attal characterized them as “acts of sabotage,” which were “prepared and coordinated.” There were no known reports of injuries.

The transport minister, Patrice Vergriete, described people fleeing from the scene of fires and the discovery of incendiary devices. “Everything indicates that these are criminal fires,” he said.

The incidents paralyzed several high-speed lines linking Paris to the rest of France and to neighboring countries, Ms. Vergriete said, speaking on BFM television.

The French national rail company SNCF said that areas affecting rail track intersections were targeted to double the impact.

“For one fire, two destinations were hit,” said the company’s chief executive, Jean-Pierre Farandou.

It was “a premeditated, calculated, coordinated attack” that indicates “a desire to seriously harm” the French people, Mr. Farandou said.

The attack occurred against a backdrop of global tensions and heightened security measures as the city prepared for the 2024 Olympic Games.

Many travelers were planning to converge on the capital for the opening ceremony, and many vacationers were also in transit.