BERLIN/FRANKFURT, March 5 (Reuters) — Tesla's European Gigafactory near Berlin halted production and was left without power after what CEO Elon Musk called an "extremely dumb" suspected arson attack that set an electricity pylon ablaze close to the site early on Tuesday.

The blaze southeast of the German capital, which did not spread to the Tesla site - the U.S. electric vehicle maker's first manufacturing plant in Europe - was extinguished by the fire brigade, police said.

A Tesla spokesperson confirmed production had stopped and the site evacuated. Tesla's shares were down 2.8% in premarket trading.

Local media published a letter purportedly from a far-left activist organisation called the Volcano Group that claimed responsibility for the incident, in a 2,500-word attack on Tesla and its billionaire CEO Musk.

Police said they were aware of the letter, which was signed "Agua De Pau", the name of a volcanic mountain in the Azores, and said they were checking its authenticity.

"These are either the dumbest eco-terrorists on Earth or they're puppets of those who don't have good environmental goals," Musk said on X.

"Stopping production of electric vehicles, rather than fossil fuel vehicles, ist extrem dumm," he said, using the German for "extremely dumb".

It was the latest setback for Tesla, which has had a bumpy ride in Europe of late, facing union pressure for collective bargaining agreements in the Nordics and supply disruptions as a result of attacks on shipping in the Red Sea.

Germany has championed new big-ticket foreign investments at a time when Europe's largest economy is facing recession and grappling with higher inflation and weaker foreign demand.

"If the initial findings are confirmed, this will be a perfidious attack on our electricity infrastructure," said local Brandenburg state Interior Minister Michael Stuebgen.

"That will have consequences. Here, thousands of people were cut off from basic services and put in danger," he said, adding that the state would react with the "utmost severity". He cautioned however against jumping to conclusions about possible perpetrators.

Tesla and local officials will give a briefing at 1430 GMT.

'We sabotaged Tesla'

Police are investigating the possible arson attack in the area around the plant, which has been the focus of environmental protests since it was launched by Musk two years ago.

They would not confirm media reports that bomb disposal units had been deployed after emergency services found a sign saying "ordnance buried here".

Workers for energy company E.ON are repairing the damage to the high-voltage pylon which knocked out power in the area, the company said.

Electricity to surrounding communities was restored except for a large industrial site and a logistics centre, it later said, but declined to elaborate.

Brandenburg state has documented previous arson attacks by far-left activists, including at a Tesla power supply site at Gruenheide in May 2021.

"We sabotaged Tesla," read the letter about Tuesday's incident, posted on website kontrapolis.info, describing the attack as a gift marking International Women's Day on March 8.

"Tesla consumes earth, resources, people, workers, and in return spits out 6,000 SUVs, killer cars and monster trucks each week."

Tesla's ambitions to expand its plant, which has a capacity of around 500,000 cars a year, hit a roadblock when local residents voted down a motion to fell trees to enlarge it.

The U.S. EV maker wants to double the site's capacity to 100 gigawatt hours of battery production and 1 million cars per year, setting it up to dominate the European market.

Environmental activists have built and occupied tree houses in the forested area they expect to be cleared if the expansion goes ahead.

Citizens in Gruenheide last month voted against a motion to clear forest for Tesla to build extra infrastructure like a train station and warehouses, leaving the decision to local authorities.

The plant's production ramp-up has slowed, though the carmaker produced 6,000 cars in a week for the first time in January.

(Reporting by Nette Noestlinger, Rachel More, Christoph Steitz, Thomas Escritt, Tom K?ckenhoff and Philipp Krach; Writing by Matthias Williams; Editing by Bernadette Baum, Jan Harvey and Mark Potter)