An extensive blackout has struck Tokyo, affecting some government offices and halting trains in the Japanese capital.

Tokyo Electric Power Co., a utility serving the Tokyo region, said officials are investigating the cause of Wednesday's blackout that broke out in the afternoon.

The company, known as TEPCO, said smoke was detected at its power substation in Niiza city, near Tokyo, and officials are looking into the cause of the incident and whether it's related to the blackouts.

Footage on Japanese television showed black smoke billowing into the sky at the substation in a business district. There were no immediate reports of injuries from the incident.

TEPCO said as many as 350,000 households and offices were temporarily out of power. Electricity was restored to most of them later.