A massive power outage that has affected tens of thousands of Berlin residents continued into its second day, with a full restoration not expected until Thursday evening.

The outage — caused by a suspected arson attack on two electricity pylons — continues to affect around 20,000 households and businesses in the southeast of the German capital, down from 50,000 the previous day.

National security authorities are investigating the event, with police suspecting it may have been politically motivated. A letter has emerged on the website Indymedia with anonymous self-described “anarchists” claiming responsibility for the attack, directed at the “military-industrial complex” of companies operating in the area. The letter hasn’t been officially verified yet.

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Berlin’s grid operator said the damaged cables are currently unusable, and that efforts are being made to reroute power by connecting various alternative cables. Works to dig up links from underground continued all night and mounting the new ones is “very complex and takes several hours,” the operator said in an email.

In the meantime, customers who have had power restored are being urged to reduce electricity use to stabilize supply. Berlin’s fire department warned residents against cooking indoors with grills or camping equipment and to unplug electronic devices to prevent damage once electricity is restored.

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The disruption has also hit local public transport. While affected train services have resumed, some tram operations remain suspended. Several schools remain closed, and emergency numbers are still unavailable in parts of the city.