SÃO PAULO—Power blackouts rolled through São Paulo, Rio de Janeiro and at least eight other Brazilian states on Monday, stoking fears that the nation’s severe drought has entered a damaging new phase.

A spike in energy use, coupled with transmission troubles, forced Brazil’s national grid operator, ONS, to order utilities to cut electricity to parts of the heavily populated Southeast. The region is suffering through sweltering summer temperatures and low reservoir levels at hydroelectric dams. Power was also cut in the nation’s capital of Brasília.

Energy Minister Eduardo Braga blamed the outage on a failure in transmission equipment. “If it wasn´t for the equipment failure, there wouldn´t have been loss of energy,” Brazilian media quoted Mr. Braga as saying.

ONS couldn’t confirm how many consumers were affected by the outages, which started around 3 p.m. and lasted nearly an hour. The affected states are home to more than 120 million people.

In São Paulo, the blackout disrupted a subway line, forcing the temporary closure of at least two stations. Commuters posting on social media said they had to press emergency-exit buttons on subway cars and walk alongside the rails to safety.