The French president, Nicolas Sarkozy, has called for those who sabotaged his country's high-speed TGV rail network to be punished with "extreme severity".
President Nicolas Sarkozy has urged protesters to go back to work
Mr Sarkozy asked the justice minister to carry out all necessary inquiries into what the state rail operator, SNCF, called a "concerted campaign".
Arsonists burnt tracks and signals, causing delays to services already hit by an eighth day of transport strikes.
Earlier, talks were held to try to end the dispute over economic reforms.
Managers from SNCF and the Paris public transport operator, RATP, held lengthy meetings with union and government representatives in the capital to try to reach a compromise.
There has been no word of a breakthrough, however, and workers are due to vote on Thursday on whether to continue a strike which the government says is costing France hundreds of millions of euros a day.
The government has vowed not to back down on its core proposal to reform the "special" pension system.
In a statement issued on Wednesday morning, the SNCF said there had been "several acts" occurring "at the same time" overnight on lines running north, west, east and south-east out of Paris.
It said they 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 asked the police to "make sure the perpetrators were punished with the most extreme severity", the secretary of state in charge of transport, Dominique Bussereau, told France 2 TV.
Prime Minister Francois Fillon later blamed militant unionists for the "criminal acts".
"The matter has been referred to the judicial authorities. Inquiries are under way, and penalties will be very severe," he told reporters.
Mr Fillon said the perpetrators of the sabotage had "no doubt" thought they could interrupt negotiations and the resumption of rail services by the SNCF, which he said was "under way".
Cables were blackened by apparent acts of arson on the railway
"Well, let me tell them that they have made a big mistake because, on the contrary, this irresponsible strategy makes negotiations and an end to the strike, which is under way, even more necessary," he added.
The prime minister finished by saying it was "high time for these strikes to stop" and for transport networks to resume.
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 railway union, suggested the acts could have been aimed to discredit the strike movement.
The unions later held three-way talks with the management of the SNCF and RATP and government representatives in Paris.
After the meeting, the head of the CGT's branch at the RATP, Gerard Leboeuf, called on transport workers to "take account of public opinion and preserve their forces to have a bearing on the talks if necessary".
'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
Nevertheless, Mr Leboeuf said union leaders would not call for an end to the strike, saying workers would be allowed to vote on whether to continue on Thursday.
"We're not going to play the role of fire fighters for this pyromaniac government and it's the workers themselves who are going to decide the next step," he added.
The next round of talks with the RATP is scheduled for Monday.
President Sarkozy has urged protesters to go back to work, saying the strike had "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, the SNCF had estimated there would be slightly improved rail services on Wednesday as the number of strikers steadily declined.
The SNCF claimed that only 22.8% of its staff remained on strike, while the RATP said 16.4% were still refusing to work.
The week of strikes has caused havoc for millions of commuters across France.
Businesses have started complaining that the strikes are hurting their operations.
The president of the Medef employers' association has described the strike as a "catastrophe" of "probably gigantic" cost to the economy.
Finance Minister Christine Lagarde has meanwhile said the dispute is costing France up to 400m euros (£290m) a day in lost business.
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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