German leftwing militants protesting over the climate crisis and AI have claimed responsibility for an arson attack that cut power to tens of thousands of households in Berlin.

The fire that broke out on a bridge across the Teltow canal in the south-west of the capital early on Saturday could deprive up to 35,000 homes and 1,900 businesses of electricity – and in many cases heat – until 8 January, the grid company Stromnetz Berlin said.

As state security authorities opened an investigation into the cause of the blaze near the Lichterfelde heat and power station that damaged several high-voltage cables, the Vulkangruppe (Volcano Group) said it had deliberately targeted some of the city’s wealthiest districts.

In a 2,500-word pamphlet seen by the Guardian which a police spokesperson called “credible”, the group said it had aimed to “cut the juice to the ruling class”.

It condemned a “greed for energy” produced by high-emission fossil fuels, calling the attack “action in the public interest” and an “act of self-defence and international solidarity with all those who protect the earth and life”.

In the message, which included details about the fire, Vulkangruppe said that datacentres used for AI were exacerbating the problem of climate-damaging energy consumption while creating dangers to society.

“We are contributing to our own surveillance and it is comprehensive. The tech corporations are in the hands of men with power which we give them,” the group said. “One day we will simply sit before bright screens or dead machines while dying ourselves of thirst and hunger.”

Cars and pedestrians pass a square where lighting has been set up using a power generator in the Steglitz-Zehlendorf district of Berlin. Photograph: Lisi Niesner/Reuters

The group said it wished to apologise to the less well-off who had been affected by the power cut, but said its sympathy was limited “for the many villa owners” now in the dark.

The area affected by the power cut includes many elderly care homes and hospitals as well as high-rise buildings with residents who are reliant on elevators that are now out of order. Berlin received moderate snowfall at the weekend and night-time temperatures plunged well below freezing.

Berlin’s mayor, Kai Wegner, said he was convinced the mass blackout had a political motive, while security experts said it bore the hallmarks of previous far-left attacks.

“It is unacceptable that leftwing extremists are once again openly attacking our electricity grid and thereby endangering human lives,” Wegner told reporters during a visit to an emergency shelter for those without heat and hot water.

With mobile networks down, police used loudspeakers attached to their vehicles to inform the public as they assisted people in need of care. Several regional commuter lines were interrupted and many pupils who were to return to school on Monday may see their holiday extended well into the week.

A previous arson attack attributed to the far left in September knocked out electricity for 60 hours in the south-east of the city in what was reportedly Berlin’s longest power cut since the second world war.

Preliminary theories for the weekend power outage had included possible Russian sabotage, as Germany has been on high alert for attacks on its infrastructure from foreign actors.

In March 2024, Vulkangruppe claimed an arson attack against a pylon at the Tesla gigafactory outside Berlin that cut off electricity to the plant and temporarily halted production.

In its 2024 annual security report, Germany’s domestic intelligence watchdog cited repeated attacks on the regional power grid by Vulkangruppe, which it classes as a leftwing extremist organisation.

It said the group, believed to be about 14 years old, chose targets for their “palpable effects on the population”, intending to cause destruction requiring painstaking and lengthy repairs with little concern for collateral damage.

Stromnetz Berlin said that initially 45,000 households and 2,200 businesses in the districts of Nikolassee, Zehlendorf, Wannsee and Lichterfelde had been affected by the power cut that began at about 6am on Saturday.

By midday on Sunday, the company said on its crisis page on its website that about 10,000 households and 300 businesses in Lichterfelde had had their electricity restored.

But cold weather was hindering efforts to lay new underground cables, meaning the remaining households may have to wait until Thursday afternoon to get their light and heat back on.

“Our teams are on the ground and we are working flat out to restore supplies,” Stromnetz Berlin said.

Experts said attacks on energy infrastructure required little specialist knowledge to have a dramatic impact, with maps featuring key components of the power grid widely available online and few contingency plans ready to be enacted in case of sabotage.