France's high-speed rail network was hit by arson attacks that disrupted travel for hundreds of thousands of passengers on Friday, just hours before the opening ceremony of the Paris Olympics.
A source close to the investigation told AFP the attacks were coordinated acts of "sabotage".
"This is a massive attack on a large scale to paralyse the TGV network," SNCF told AFP, adding that many routes would be cancelled.
"SNCF was the victim of several simultaneous malicious acts overnight," the national train operator said, adding that the attacks affected its Atlantic, northern and eastern lines.
"Arson attacks were started to damage our facilities," it said, adding that traffic on the affected lines was "heavily disrupted" and the situation would last through the weekend as repairs are conducted.
SNCF chief executive Jean-Pierre Farandou said 800,000 passengers were affected.
Transport Minister Patrice Vergriete called the attacks an "outrageous criminal act" that would have "very serious consequences" for rail traffic throughout the weekend.
He said connections towards northern, eastern and northwestern France would be halved.
SNCF said trains were being diverted to different tracks "but we will have to cancel a large number of them".
The southeastern line was not affected as "a malicious act was foiled".
SNCF urged passengers to postpone their trips and stay away from train stations.
The attacks were launched as Paris was under heavy security ahead of the opening ceremony of the Summer Olympics, with 300,000 spectators and an audience of VIPs expected at the event.
The parade on Friday evening will see up to 7,500 competitors travel down a six-kilometre (four-mile) stretch of the river Seine on a flotilla of 85 boats.
It will be the first time a Summer Olympics has opened outside the main athletics stadium, a decision fraught with danger at a time when France is on its highest alert for terror attacks.
At Paris's Montparnasse train station, passengers were waiting for more information about their trips, with display boards showing delays of more than two hours.
"Normal traffic is expected to resume on Monday, July 29," read one of the signs in the departure hall.
The station's loudspeakers told passengers that conditions to exchange and refund tickets would be more flexible.
Graphic designer Katherine Abby, 30, clung to hope that her trip would only be delay and not cancelled. She booked her tickets for Biarritz, a popular southwest beach resort, weeks ago.
"It's my only vacation of the year, said Abby, who was travelling with her husband.
"I've been waiting for this moment for a year, I would be pretty demoralised to have to cancel this trip, especially when you see what Paris looks like with the Olympic Games," she said.
