France's national rail operator on Saturday said train travel was gradually returning to normal after multiple sabotage acts paralysed the network hours ahead of the Olympics opening ceremony.

It remains unclear who carried out the three attacks overnight Thursday to Friday on rail infrastructure and if they were deliberately timed to disrupt the Olympics opening ceremony that took place later that day.

However, a message of support for the sabotage was sent to several media outlets on Saturday, which also criticised the Olympics.

Tens of thousands of rail passengers struggled through a second day of cancelled and delayed trains on Saturday with 160,000 of the 800,000 people due to travel on a major holiday getaway weekend still facing cancellations.

Transport Minister Patrice Vergriete said national rail operator SNCF had mobilised a hundred workers overnight to repair the damage "in difficult weather conditions" and ensure a return to normal for Monday.

During the night, "with floodlights, in the rain, they patched up the wires one by one," said SNCF chief executive Jean-Pierre Farandou.

"Everything will be repaired by Monday morning" on the SNCF network, he added.

Fibre optic cables running near the tracks and ensuring the transmission of safety information for drivers, such as signalling lights and points, were cut and set on fire during the night from Thursday to Friday on three of the main TGV lines, in the west, north and east of France.

No claim of responsibility has been made for the meticulously planned night-time attacks. Maintenance workers thwarted a fourth attack.

But a statement sent to several news media expressed support for the sabotage and criticised the Olympic Games.

"They call it a festival? We see it as a celebration of nationalism, a gigantic stating of the subjugation of populations by states," said the email received by the newspaper La Provence.

Although the headline of the message said, "claim to sabotage," no details of the action were provided.

It was signed by "an unexpected delegation" and was sent via the online platform Riseup, whose users include those advocating for social change.

Paris prosecutors on Friday opened a probe into an attack on "fundamental national interests."

Observers have noted that the nature of the attacks indicate the culprits would have needed deep knowledge of the functioning of the railways.

Interior Minister Gerald Darmanin said the investigation was progressing, as he also expressed "relief" that the opening ceremony had taken place without any mishap.

"We have uncovered a certain number of elements that allow us to think that we will soon know who is responsible for what clearly did not sabotage the Olympic Games but did sabotage part of the holidays of the French people," Darmanin told France 2 television.

He said it was "too early" to say who was behind the attack. "It could have come from inside (France) or from the outside."

French authorities are on high alert for a terrorist attack during the Games, which run through August 11. Tens of thousands of police and troops are on security duties during the Olympics.

Some 250,000 people missed their train on Friday, according to SNCF.

About three out of every 10 trains were cancelled on Saturday in the three regions affected by the attacks, with most trains still operating delayed by between one and two hours, SNCF said.

Nearly one third of trains were cancelled in northern, western and eastern France. About a quarter of Eurostar high speed trains between Paris and London and Paris and Brussels were also called off.

With normal services set to be fully restored on Monday, the situation is already expected to improve markedly on Sunday.

Nine out of 10 TGV high speed trains will be able to run on the western and southwestern lines on Sunday, while service will be more patchy in the north, where seven out of 10 trains are due to run, SNCF said. Trains to eastern France have not been affected. (AFP)