The electricity went out on Tuesday afternoon, local time, and there is still no forecast for when the power supply will be fully restored.
Authorities are blaming a fault in the power grid, but the exact cause of the outage is not known. The fault is said to have started with one power plant being shut down, after which other power plants could no longer cope with the increased strain.
The vast majority of hospitals have backup generators, but subways and trams are at a standstill and the streets have been filled with people trying to get home on foot.
