Before dawn on the day of the opening ceremony of the Paris Olympics, arsonists on Friday sabotaged three critical high-speed rail lines, stranding thousands of travelers, heightening security fears and blighting what President Emmanuel Macron hoped to be a moment of national glory.

Around 4 a.m., the arsonists cut through and burned cables used for signaling and security near three rail divides, the French authorities said. The carefully placed strikes snarled end-of-week travel plans for more than a million people, including Parisians leaving for vacation and international travelers en route to the opening ceremony of the Games. Parts of the prized rail system came to a standstill.

A fourth sabotage attempt was foiled, the authorities said, when railway workers doing maintenance work in Vergigny, southeast of Paris, stumbled upon suspicious individuals who fled before any damage was done.

No one was killed or reported injured. The Paris prosecutor’s office, which handles major organized crime cases, said it had opened an investigation into what it called criminal vandalism and criminal conspiracy. No immediate claim of responsibility was made.

“This operation was prepared, coordinated, critical points were targeted, which shows they knew enough about the network to know where to strike,” Prime Minister Gabriel Attal said.

He added, “All of our services are obviously mobilized to organize the games so that they are a success.”