Environmentalists opposed to the expansion of Tesla's massive Gruenheide car plant, near Berlin, Germany are thought to have been behind suspected sabotage on Tuesday that disrupted electricity supply to the factory and to parts of the nation's capital.

The plant, which produces around 500,000 electric vehicles a year, had been earmarked for expansion but the project was dealt a blow last month when local residents voted against it in a non-binding referendum, largely because of the 100 hectares of forest that would be lost as a result.

Tesla wants to double the Gruenheide factory's capacity, to 100 gigawatt hours of battery production and to the production of 1 million electric vehicles a year, with aspirations of becoming the most dominant force in Europe's electric vehicle market.

But several local groups oppose the plans.

The Reuters news agency said police heard about Tuesday's fire, which caused the plant to halt production temporarily, in an early-morning phone call from the suspected saboteurs.

Germany's tabloid BZ newspaper added that the fire in the Gosen-Neu Zittau area, a few kilometers southeast of Berlin, enveloped an electricity substation and that bomb disposal teams attended the scene alongside firefighters.

Later on Tuesday, a letter claiming to be from an organization called the Volcano Group was published online saying the sabotage was carried out to highlight the company's supposed heavy use of resources and labor, and its alleged contamination of drinking water.

Police were not immediately sure about the letter's authenticity.

The BBC quoted Michael Stuebgen, the interior minister for the German state of Brandenburg, as saying: "The rule of law will react to such an act of sabotage with the utmost severity."

The Associated Press said Tesla has been studying its options in the wake of last month's non-binding referendum defeat, but still hopes to expand the plant.

The "Berlin gigafactory" delivered its first cars to customers in March 2022, joining Tesla's two other major facilities: in California and Shanghai.

Germany's leader, Chancellor Olaf Scholz, hailed the facility as being at the "forefront of industry" but the plant, and Tesla in general, has been facing growing opposition in recent months, according to AP, with protesters setting up camps in the forests around the factory, from where they held a piano concert on Saturday and handed out cake to draw attention to their opposition. Last week, a local government even claimed the car plant's activities were polluting local drinking water.

Lou Winters, from the campaign group Tesla den Hahn Abdrehen, which translates as Turn off Tesla's Tap, told AP: "Tesla brought a lot of noise and air pollution in the region because of all the trucks that drive every day over the streets. I talk to people who moved some years ago to Grunheide (the municipality where the plant is located) for the fresh air, for the calmness. Now almost nothing is left of this."

The friction at the German plant is part of a series of recent challenges Tesla has had in Europe; with unionized workers in Scandinavian plants becoming increasingly belligerent, a privacy investigation looming in the Netherlands, and the European Union threatening to clamp down on car imports in a bid to support domestic manufacturers.