The power cut plunged the capital Caracas into almost complete darkness during rush hour on Thursday, before extending to other areas.
The government of President Nicolás Maduro has blamed the opposition, accusing them of sabotage.
It comes amid rising tensions over opposition efforts – backed by the US and some Latin American countries – to remove Mr Maduro from power.
The lack of electricity has caused flights to be diverted from the main airport in Caracas, where thousands of workers were forced to walk home.
Venezuela depends on its vast hydroelectric infrastructure, rather than its oil reserves, for its domestic electricity supply.
However, decades of underinvestment have damaged the major dams, and sporadic blackouts are commonplace.
Mr Maduro accuses opposition leader and self-declared interim president Juan Guaidó of trying to mount a coup with the help of “US imperialists”.
Mr Guaidó said the blackout was a matter of “chaos, concern and anger” and “evidence of the usurper’s inefficiency”.