Saboteurs struck France's TGV high-speed train network in a series of pre-dawn attacks that caused chaos on the country's busiest rail lines ahead of the Paris Olympics opening ceremony on Friday (July 26).

The coordinated sabotage took place as France rolled out an unprecedented peacetime security operation involving tens of thousands of police and soldiers to lockdown the capital for the Games, sucking in security resources from across the country.

The state-owned railway operator said vandals had damaged signal boxes along the lines connecting Paris with cities such as Lille in the north, Bordeaux in the west and Strasbourg in the east. Another attack on the Paris-Marseille line was foiled.

READ: France's high-speed railway hit by 'sabotage' hours before Olympics

Hundreds of thousands of people were left stranded at rail stations.

There was no immediate claim of responsibility. But two security sources said initial suspicions fell on hardline leftist militants or environmental activists.

More than 300,000 spectators are expected to line the banks of the River Seine when the athletes parade through the heart of Paris on a flotilla of barges and riverboats, part of an extravagant opening ceremony that will be watched by a global audience of billions.

The SNCF urged all travelers to postpone their journeys. Repairs were underway but traffic would be severely disrupted until at least the end of the weekend. Trains were being sent back to their points of departure. The attacks hit signaling installations on the Atlantic, Northern and Eastern high-speed lines with fires set off by explosive devices, the SNCF said.

SNCF chief Jean-Pierre Farandou said some 800,000 customers had been impacted ahead of a busy weekend for French holidaymakers. Thousands of rail staff had been deployed to repair the damage.

(Production: Marco Trujillo, Stef Haskins, Ursa Presern)