According to the SNCF -- the organizer of France’s high-speed train network -- the "malicious acts" took place overnight Thursday ... and appeared to be a planned "massive attack" by people who had the goal of crippling public transportation just hours before the Games' Opening Ceremony goes down in Paris later Friday.

So far, authorities don't seem to have any leads on suspects ... although the public prosecutor's office said it's launched an investigation.

Thankfully, no injuries have been reported ... but images from train stations all over the country show it's all caused huge delays and backups for people looking to make moves ahead of the start of the Games.

French Prime Minister Gabriel Attal didn't appear optimistic things would return to normal any time soon ... writing on X, "the consequences on the rail network are massive and serious."

The SNCF, meanwhile, said it's encouraging passengers to postpone planned trips ... as repairs promise to be painstaking and arduous.

Despite the chaos ... Paris Mayor Anne Hidalgo told reporters the Opening Ceremony will still go down as planned at the Seine River.