BERLIN, Jan 7 (Reuters) - Electricity was restored to southwestern Berlin on Wednesday after a suspected arson attack on a power station by leftist activists caused a blackout for tens of thousands of households, the longest outage in the German capital since World War Two.

"Today is a good day for the many people affected who had been cut off from electricity and heat since January 3," said Berlin Mayor Kai Wegner.

The operation to restore power, which started at 11 a.m. (1000 GMT), was a complex and gradual process, he said.

A blaze early on Saturday destroyed a cable duct over a canal, cutting off power in about 45,000 households and more than 2,000 businesses in southwestern districts of the city at a time of freezing temperatures.

The far-left Volcano activist group, which in 2024 said it attacked a power pylon near Tesla's factory outside Berlin, claimed responsibility.

The German army was called in to support residents struggling with the outage, which also affected mobile phone connections, heating and trains.

The prolonged fallout has prompted some politicians to demand more investment to protect the capital's infrastructure, especially since the domestic intelligence agency has warned about growing risks from left-wing militants.

(Reporting by Madeline Chambers; editing by Matthias Williams and Jan Harvey)