A power cut has left almost half of Venezuela without electricity, including parts of the capital Caracas.
The blackout disabled traffic lights in the city, causing traffic chaos. It also partially disrupted the underground transport system.
Thousands of workers were sent home. Blackouts are common in Venezuela, but they rarely affect Caracas.
The deputy energy minister, Franco Silva, said failures in transmission lines were to blame.
Energy Minister Jesse Chacon apologised to Venezuelans and asked them to be calm.
Authorities say they are working on re-establishing services, and they will give priority to transport in Caracas.
President Nicolas Maduro blamed the opposition for the blackout. “Everything seems to indicate that the far-right has resumed its plan for an electrical strike against the country,” he said on Twitter.
But critics say poor management and the failure to invest in infrastructure are to blame.
The oil industry has not been affected by the power cut, as Venezuela’s oil refineries are powered by separate generator plants.
In 2010 the late President Hugo Chavez signed a decree declaring an “electricity emergency” to help his government tackle power shortages.
Although Venezuela has big oil reserves, it is dependent on hydro-electricity for some 70% of its power.