REPUBLIKA.CO.ID, MARSEILLE -- French police using water cannon and tear gas broke up a picket by striking workers who were blocking access the Fos-sur-Mer refinery in the southern port area of Marseille, an official of the CGT union said on Tuesday.

The pre-dawn police operation followed warnings from the government that attempts to strangle fuel supplies in a dispute over contested labour law reforms would not be tolerated for long.

It marked an escalation in the standoff between President Francois Hollande's government and the hardline CGT union, which is seeking to turn various protests and stoppages into full-scale rolling strikes at ports, oil refineries and railways.

Emmanuel Lepine of the CGT union said about 40 busloads of riot police took part in an operation "of unprecedented violence".

Workers at the Fos refinery, which belongs to U.S.-based Exxon Mobil, are taking part in a broader series of strikes and pickets in the sector.

The CGT union said strikes had now spread to all eight of France's refineries. "Output is going to fall by at least 50 percent," Lepine said.

Beyond the stoppages by refinery workers, pickets by dockers, truckers and other protesters have been blocking deliveries in and out of fuel distribution depots for days, sparking panic buying and shortages at hundreds of car fuelling stations.

The government has said there are sufficient supplies to ride out the stoppages, but French oil major Total said earlier this week that hundreds of its 2,200 petrol stations had either partly or completely run out of fuel.

The CGT union has also called rolling strikes of more than two days a week on the national railways and for an open-ended strike on the Paris subway and suburban commuter train networks from June 2, a week before the Euro 2016 soccer tournament opens.

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