Fuel deliveries have been blocked from all French refineries, according to union officials, as protesters aim to bring France to a standstill over plans to raise the pension age from 62 to 64.
"The strike has begun everywhere," said Eric Sellini of the CGT union.
Tuesday marked the sixth day of strikes and protests since mid-January, with unions saying it would be the biggest yet.
Most train and metro services have been cancelled, and many schools closed.
At least 260 demonstrations are expected across France on Tuesday, with police estimates of between 1.1 million and 1.4 million taking part.
With the government showing no sign of backing down on its pension plan, one union leader, Emmanuel Lépine, said last week that the aim of blocking fuel deliveries was to "bring the French economy to its knees".
There are eight refineries in mainland France and the hard-line CGT union said strikers had blocked the exits to all of them, including those run by TotalEnergies and Esso-ExxonMobil.
However, it was not clear how long the blockades would last.
In the days ahead, there will be calls to extend the strikes to include power generation too.
So far, for all the noise and sporadic disruption, the campaign has caused little damage to the economy, and the bill is proceeding through parliament.
Unions and the left know time is running out before the reform becomes a reality - which is all the more reason for them to up the pressure now.
President Macron has called the reform "essential" because of deficits forecast for France's pension system over the next 25 years, according to analysis by the independent Pensions Advisory Council.
As well as raising the pension age by two years, the government says workers would have to contribute for 43 years into France's shared pension fund before earning a full pension.
Very few French workers have personal pensions linked to capital investments.
Neighbouring European economies have already raised the retirement age in recent years to reflect longer life expectancy. Italy, Germany and Spain have moved towards raising the official retirement age to 67, while in the UK it is currently 66.
The majority of French citizens back the ongoing strikes, a poll by French survey group Elabe suggests.
According to the opinion poll, 56% of people support rolling strikes and 59% back the call to bring the country to a standstill.
Two-thirds of people support the protest movement against the government's planned pension reforms in general, the poll indicates.
However, the number of protesters taking to the streets began to fall last month and union leaders now believe rolling strikes are their best hope of success.
"If the refineries are blocked we could run out of petrol by the end of the week," warned Thierry Cotillard, head of French supermarket group Les Mousquetaires.
Government spokesman Olivier Véran told France 2: "We don't want French people to be, quote, unquote, 'victims' of a long-term blockade."
Last week, he warned that the strikes could lead to an "ecological, agricultural and health catastrophe".
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