Firefighters in France were responding to reports of fires that were causing delays on high-speed train lines connecting Paris to several surrounding cities on Friday morning, the opening day of the Olympics.

Patrice Vergriete, France’s transportation minister, said in a statement that “coordinated malicious acts” had targeted several lines of the country’s high-speed TGV trains. They were expected to disrupt traffic and vacationers’ travel plans into the weekend, he said.

The national railway company, known as the S.N.C.F., said that a “malicious act” had taken place around Arras, a town about 100 miles north of Paris, on the high-speed train line between Paris and Lille. Another such act took place near Courtalain, a town about 90 miles southwest of Paris, on the lines connecting Tours and Le Mans to Paris, it said. The local media reported that there had been fires on the rail lines.

Firefighters were responding to the two episodes, which happened about 5:15 a.m., the company said. Customers should expect delays of 30 to 90 minutes, and cancellations were possible, it said.

The mood was already tense in Paris, where security has been tightening like a vise for weeks ahead of Friday night’s opening ceremony. Tens of thousands of police officers, joined by counterterrorism units and the military, have been deployed across the city to guard Olympic venues, tourist sites, train stations and street corners.

Weeks ago, the authorities cordoned off a section of the Seine on both banks to protect the thousands of athletes who will take part in the ceremony in a huge flotilla of boats and the hundreds of thousands of fans who will come to see it.

This is a developing story.