Officials said they believe the outage, which struck July 23 around 11 a.m. in Spain, was triggered by a faulty cable that fell and caused problems in several substations.
Around 350,000 households and businesses lost power at some point, and more than 100,000 remained powerless that night.
Firefighters reported a flood of calls from people stuck in elevators, and police officers were sent to major intersections to direct traffic due to failed lights. Several subway lines also were affected, however, the airport was not.
Officials said at least one major hospital was relying on generator power to light operating rooms.
A city official speaking on condition of anonymity because he was not allowed to talk to the media said around 30,000 households and businesses would be without power until July 24, and that as many as 80,000 would have to wait longer, possibly weeks.
Red Electrica, which manages Spain's power grid, said more than half the power supply had been restored within four hours. The outage affected around 350,000 customers of electricity operator Fecsa Endesa, Industry Minister Joan Clos said. Around 1.5 million people live in Barcelona.
Clos, a former mayor of Barcelona, ruled out sabotage, blaming the blackout on a substation cable that fell, causing a chain-reaction failure in as many as six other substations. It also caused a fire in one substation. Clos said the reasons for the accident were being investigated.