Unfortunately, we have to hit the places where people will feel it most

Christian Mansano CGT Union spokesman

The unions organising the protest said there were around 35 blockades at oil refineries, motorway tollbooths and road junctions. Cities affected included Rennes, Lyon, Rouen, Marseille, Le Havre, Toulouse and Bordeaux.

One set of drivers also began blockading food depots outside Marseille, after police prevented them protesting at an oil refinery.

Christian Mansano, of the CGT union, said the Marseille food depots had been targeted because police had turned them away from refineries.

"We had to go where they weren't waiting for us," Mr Mansano said.

"Unfortunately, we have to hit the places where people will feel it most".

The blockades have caused some fuel shortages, although refineries like TotalFinaElf - the country's largest - said they had increased stocks at petrol stations in anticipation.

But the protests have caused havoc on the roads. Lorry drivers in Rouen, Caen and Tours in the northwest of the country are driving deliberately slowly on the roads, causing major tailbacks.

Political tensions

The drivers are unhappy that they are no longer being paid in full for all the hours they work.

Socialist PM Jospin is hoping to be elected president

They argue that the government's decision to introduce a 35-hour working week does not take into account rest breaks that truckers are required by law to take, or time spent waiting during loading and unloading.

Four unions are participating in the action, representing some 200,000 drivers, although the main lorry drivers' union has decided not to take part.

The action comes as campaigning for next month's presidential elections heats up.

Socialist Prime Minister Lionel Jospin is hoping to oust incumbent conservative President Jacques Chirac in ballots on 21 April.

Industrial unrest could damage his chances of success.