French police used water cannons to disperse projectile-throwing protesters blocking a key fuel depot on the Mediterranean.
Strikes spread to all eight of France's refineries, with one in five petrol stations across the country now dry or running low.
A two-month protest movement against a bill weakening France's famed worker protections reached a new level this week as fuel industry workers joined in. It is posing the biggest challenge yet to President Francois Hollande and his government.
Other forms of transport are unlikely to provide much relief with unions planning strikes on the SNCF rail system and Paris subways and buses later this week.
The CGT union, whose hard-left flank is driving the movement, reacted angrily to the overnight police advance on the depot at Fos-sur-Mer.
Police pushed out picketing workers gathered around burning tires. After the blockade was cleared, fuel trucks that had been stuck for days outside the depot resumed traffic, but faced long delays loading up.
Prime Minister Manuel Valls told Europe-1 radio: "Other sites will be liberated. I'm very determined."
The striking workers are equally determined, pledging to keep fighting until the government withdraws its disputed labour reforms.
The government is trying to change rules governing the working week, overtime pay and other labour protections, something opponents say will weaken protections without promoting job creation.
The junior minister for transport, Alain Vidalies, said on television that 20% of petrol stations around the country are facing shortages as of Tuesday morning and unions are threatening to stop production at all of France's eight refineries.
Consumers, meanwhile, are flocking to the pumps in anticipation of further problems. Images were shown on television of French drivers even crossing the border to fill up in Belgium.