About 1,500 petrol stations in France have run dry or are about to close as fuel supplies are hit by strikes over government pension reforms, officials say.
A blockade of oil refineries has lasted a week and the body that supplies most supermarkets says one in four petrol stations is affected.
President Nicolas Sarkozy has called a crisis cabinet to protect supplies.
He told reporters that the reforms were "essential" and would be carried out.
The exact number of France's 12,000 petrol stations affected by the strikes is unclear, but oil company Exxon Mobil has described the situation as "critical".
A spokeswoman said that anyone looking for diesel around Paris or in the western area of Nantes would face problems.
Severe shortages have been reported in Brittany in north-west France and the International Energy Agency says that France has begun tapping into its emergency oil reserves.
Workers at France's 12 oil refineries have been on strike for a week and entrances to many of the country's fuel distribution depots have been blocked.
Panic-buying was blamed for a 50% increase in fuel sales last week.
The head of the Leclerc chain of supermarkets, Michel Edouard Leclerc, warned that at the current rate his company's petrol stations would be empty within two to three days if the blockade of refineries remained and fuel imports were paralysed.
Strike action against the government's reform plans is being ramped up, with lorry drivers starting the week by staging a go-slow on motorways around several major cities including Paris, Lille and Lyon.
A further day of strikes is scheduled for Tuesday, on the eve of a key Senate vote on the pensions bill.
Half of all flights to and from Paris's Orly airport and one in three flights at other airports are being cancelled, according to aviation officials.
Airport operator ADP said there were already some delays at the capital's largest airport, Charles De Gaulle, on Monday because of strikes by oil workers.
Street protests have been planned in a number of cities and disruption is also expected on public transport and in schools.
The government wants to raise the retirement age from 60 to 62 and the full state pension age from 65 to 67.
There were already demonstrations outside 261 schools on Monday, which the education ministry said had been blockaded.
In the western suburb of Nanterre in Paris, dozens of students clashed with riot police who fired rubber bullets.
Shop windows were reported broken in the Saint-Denis suburb, where education officials said more than half the areas secondary schools had been blockaded.
In Lyon, several cars were burned and one teacher whose car was badly damaged by fire complained: "They want to fight [against the pension reform]. OK, but they have to understand the meaning of what they are doing".
In other developments:
- Outside the Grandpuits refinery, east of Paris, strikers burned tyres in defiance of a government order that the facility should be reopened.
- In some cities, such as Toulouse (south-west) or Saint-Etienne (centre), public transport depots were blocked on Monday morning, preventing buses and tramways from operating for several hours.
- Rail traffic was being disrupted with one in two fast TGV trains running, and one in three normal-speed trains running.
- Although the Eurostar train service between Paris and London is normal, there is no Eurostar service between Brussels and London on Monday due to a strike in Belgium.
- A key fuel pipeline that supplies the two airports in Paris has been restored, but the civil aviation authority is warning airlines operating at Charles de Gaulle to arrive with enough fuel to make the return journeys.
French President Nicolas Sarkozy has ordered key ministers to form a crisis cabinet with the role of ensuring the continuity of fuel supplies.
Three departments are being charged with coordinating the state's services to maintain the supply: the interior and economy ministries as well as the energy and environment department.
Prime Minister Francois Fillon has insisted he will not allow the refinery strikes to hit the French economy.
Several other figures have said the country is not at risk of fuel shortages.
"The government is in control," Industry Minister Christian Estrosi told French radio on Monday.
"There will be no blockade for companies, no blockade for transport and no blockade for road users."
The head of the French Petrol Industries Association, Jean-Louis Schilansky, has said fuel shortages are not yet at crisis point.
"If the lorry drivers go on strike, if people block the refineries, then we will have a very big problem. But we're not at that stage yet," he said.
France has a strategic fuel reserve which holds up to three months of supplies, the government says.
According to the latest opinion polls, more than 70% of French people continue to support strike action.
On Saturday, a fifth day of protests brought 825,000 people on to the streets, police said, although unions put the figure at 2.5 million to three million.
The pension reforms have already been approved by the National Assembly, the lower house of the French parliament.
The upper house, the Senate, has endorsed the key articles on raising the retirement age, and is due to vote on the full text on Wednesday.