FRANKFURT, Jan 4 (Reuters) - A fire in southwest Berlin that has left tens of thousands in the German capital without electricity was likely the result of a far-left extremist attack, officials said on Sunday.

Grid company Stromnetz Berlin said on Saturday that the suspected arson attack could leave up to 45,000 households without power until January 8.

On Sunday, it said efforts to restore power were ongoing, with around 35,000 households and 1,900 commercial entities still affected.

Following the attack, local media published a letter purportedly from a far-left activist organisation called the Volcano Group that claimed responsibility for the incident, saying its actions were directed at the fossil-fuel based energy industry.

"The letter claiming responsibility has been classified as authentic by the security authorities," Iris Spranger, Berlin's interior affairs minister, said in a post on x.

"I condemn this inhumane attack on Berliners and visitors to the city in the strongest possible terms. The investigation is ongoing."

In September, a suspected arson attack on two pylons left around 50,000 households in Berlin without power in an incident local media said bore similarities to Volcano Group's high-profile attack on the power supply of Tesla's gigafactory in Gruenheide in 2024.

(Reporting by Christoph SteitzEditing by Christina Fincher)