France's high-speed TGV rail network has been damaged by a "concerted campaign of sabotage", the SNCF state-owned rail operator has said.
Cables were blackened by apparent acts of arson on the railway
It said acts of sabotage overnight, including fires, caused huge delays to TGV services already hit by a transport union strike in its eighth day.
President Nicolas Sarkozy said culprits would be severely punished. Union leaders also condemned the attacks.
The sabotage came as three-way talks opened in a bid to end the strike.
Managers from SNCF and the Paris metro operator RATP were meeting unions and government representatives to try to reach a compromise on reforms to transport workers' pensions.
The government has vowed not to back down on its core proposals.
In a statement, the SNCF said there had been "several acts" occurring "at the same time" on lines running north, west, east and south-east out of Paris.
It said these included a "very large" fire on the TGV's Atlantic branch that damaged signals affecting 30km (18 miles) of track.
At a cabinet meeting, President Sarkozy ordered the justice and interior ministers to investigate, vowing to punish the perpetrators "with the utmost severity", Transport Minister Dominique Bussereau told France 2 TV.
Union officials also deplored the attacks as acts of vandalism by "cowards", warning that they put people's safety at risk.
Bernard Thibault, chief of the powerful CGT union, suggested the acts could have aimed to discredit the strike movement.
The TGV network was targeted several hours after French Labour Minister Xavier Bertrand said he hoped that conciliation talks would help end the strike.
'SPECIAL' PENSIONS SYSTEM
Benefits 1.6m workers, including 1.1m retirees
Applies in 16 sectors, of which rail and utilities employees make up 360,000 people
Account for 6% of total state pension payments
Shortfall costs state 5bn euros (£3.5bn; $6.9bn) a year
Some workers can retire on full pensions aged 50
Awarded to Paris Opera House workers in 1698 by Louis XIV
The talks opened on Wednesday.
"I think the conditions are there for everyone to get out of it honourably," Mr Bertrand was quoted as saying by the AFP news agency.
President Sarkozy also urged the protesters to go back to work now that negotiations were beginning.
"Everyone must ask whether it is right to continue a strike which has already cost users - and strikers - so dear."
The government has said there could be incentives of salary rises and a top-up scheme for pensions.
But it has stressed that there will be no budging on the core issue of eliminating special pensions which allow 500,000 transport and utility workers to retire early.
Didier Le Rester of France's General Labour Confederation has predicted that the negotiations could last up to a month.
Before the latest incidents, SNCF had estimated there would be slightly improved rail services on Wednesday as the number of strikers steadily declined.
SNCF now claims that only 23% of its staff remain on strike.
The week of strikes has caused havoc for millions of commuters across France.
Businesses have started complaining that the strikes are hurting their operations.
Finance Minister Christine Lagarde said the dispute was costing France up to 400m euros (£290m) a day.
Very large fire reported to have damaged signal equipment affecting 30km of track on the Atlantic line
Signal switches in the South East and East lines reported to have been sabotaged
Fire reported to have damaged signal cables affecting the North line
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