Hundreds of petrol stations have been closed across France
French police have fired tear gas at masked protesters in Paris as protests against the government's planned labour reforms turned "ugly".
Reporter Jonathan Samuels said: "They're throwing stones, rock, missiles, bits of plaster that have been chipped off walls of apartments and office blocks here. They're lobbing them at police and police are retaliating".
"There's a to-and-fro battle between a hardcore group of troublemakers and the police".
At one point a protester managed to spray a number of riot police and their shields with pink paint. The officers responded by charging at the crowd.
Police had earlier detained nine people and scuffled with masked protesters as several thousand people marched through the French capital to demand the scrapping of the proposals.
The rally in Paris is part of a national day of action against plans to loosen protections for French workers.
Thousands of demonstrators are on the streets across the country to protest against the changes, and they have been tightening their grip on France's fuel supplies.
Petrol stations are running dry, tankers are being held off the southern coast and not allowed to dock, the government is dipping into precious reserves and nuclear power capacity has been cut because workers have walked out.
There are also reports of train lines being obstructed.
And there are signs of pockets of panic-buying, with some people saying they are filling up their cars in case the protests go on.
One petrol station manager, Bernard Ballaux, said: "The volume of fuel that was bought by clients increased 40% since Monday".
After weeks of road blocks and pockets of violence, as well as days of logjams outside refineries to stop fuel getting out, demonstrators are turning their attention to the capital.
Union members say it could be the biggest coordinated demonstration of the unrest so far.
Corinne Besson, who lives near Le Havre in northern France, said she had to wait in a queue twice on Wednesday to fill her tank.
"I waited in line for 40 minutes to fill up 20 litres, and now I'm waiting in line again. I think it'll take about an hour and I'll probably get about 20 litres again".
According to Google, the top search term on the internet in France is "how to siphon petrol".
Among the roads blocked were the Pont de Normandie and the Pont de Tancarville - the main routes from the port of Le Havre heading south towards Normandy and Paris.
These routes have reportedly re-opened but the dock city has been stormed by workers throwing smoke bombs.
There are fears the action, led by the CGT Union, could carry on until the Euro 2016 football championships.
The French government is standing firm, refusing to abandon the new labour laws which make it easier for employers to hire and fire workers in a country where unemployment is running at 10%.
"Unfortunately democratic choice has been completely put aside," said demonstrator Damien Girard.
"The unions were not consulted, the discussion procedures at the national assembly were disregarded as well, so we chose to strike in order to make ourselves heard and it ended up paralysing the nation's economy".
French Prime Minister Manuel Valls said: "What are our options? Changing the bill? That's just not possible".
Speaking to BFMTV he added: "It's out of the question to change the current outlines. There may always be changes and improvements. Often this country kills itself with conservatism, and with its incapacity to reform".
"It's not about standing your ground on this - it's about the text being good for employees, companies and the unions".
Mr Valls has conceded that there could be moves towards concessions but has not talked about the potential details.