Electricity has been restored to thousands of households in the southwest of the German capital, Berlin, after a power outage hit some 45,000 homes and 2,220 businesses on Saturday, the grid operator said on Sunday.

The local electricity operator Stromnetz Berlin reported that power has been restored to about 14,000 households and nearly 500 commercial customers.

Several large hospitals were also back online.

Earlier, Stromnetz Berlin said it could take until Thursday afternoon to remedy the problem completely with thousands remaining without electricity in the district of Steglitz-Zehlendorf.

The outage, which leaves many of those homes without heating, comes as the city is experiencing a bout of very cold winter weather.

Mayor calls attack on energy infrastructure 'terrorism'

Berlin Mayor Kai Wegner on Sunday classified the attack on the city's power infrastructure as terrorism, saying it went beyond arson or sabotage.

"This is already terrorism," the conservative politician of Chancellor Friedrich Merz's Christian Democrats (CDU) told regional broadcaster RBB.

He said a left-wing extremist group targeted critical infrastructure and endangered lives, including elderly people relying on medical equipment, and families with young children.

"We must catch these perpetrators now," Wegner said

The loss of power was caused by a fire at a cable link over the Teltow Canal that police are investigating as a possible arson attack after a letter sent to authorities claimed responsibility.

"We are also in contact with the Federal Criminal Police Office, with the federal level, because this is no joke but a terrorist attack," Wegner said.

The affected district contains several care homes and medical facilities. Emergency services have transferred residents from at least two homes for the elderly.

A spokeswoman for Berlin's Senator for Economic Affairs Franziska Giffey said authorities believed that a malicious act was behind the fire.

A Berlin police spokesman also said that it was "credible" that the letter was written by a left-wing extremist group calling itself the "Vulkangruppe" (Volcano Group), as the letter claimed.

He said, however, that the investigation was still ongoing.

A document titled "Cutting off the Power to the Rulers" was received from the alleged perpetrators claiming responsibility for Saturday's attack saying the gas power plant in Berlin-Lichterfelde was "successfully sabotaged."

"Power outages were not the target of the action, but rather the fossil energy industry," the group claims in the letter.

Police call for better surveillance

The head of the Berlin branch of the GdP police union, Stephan Weh, called the attack "senseless" and without any legitimation, "even if the suspected arsonists again throw together all sorts of hypocritical reasons in a pamphlet on the end of the world."

He called for an "unideological" debate on video surveillance in view of the fact that so many thousands of people were affected.

"Cameras mounted on electricity poles could help investigations but also have a preventive effect. We do without protective technology in the public space just because some people cry out loudly and cite Orwell's '1984,'" he said. 

His words were echoed by Alexander Schirp, the director of the regional business associations in Berlin and Brandenburg (UVB).

"We are expecting damage costing millions to plants and machines and owing to high losses in revenue," he said, calling for better protection of infrastructure against attacks and accidents.

"This is a serious problem and stokes a feeling of insecurity in the business world," he said.

Stromnetz Berlin said there had been a similar attack on two power pylons in the city's southeast in September that was politically motivated.

Edited by: Sean Sinico

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