'Answers urged'

The outage on Tuesday evening began when the Itaipu hydroelectric dam, which supplies much of the country's electricity, suddenly went offline, plunging parts of Rio de Janeiro, Sao Paulo and other Brazilian cities into darkness.

Gabriel Elizondo, Al Jazeera's correspondent in Sao Paulo, said: "If you ask people on the streets of Rio or Sao Paulo, they are caring about the fact that their lives were very much affected last night and this morning. They don't want a repeat of this.

Police across Brazil were put on alert amid fears of a crime wave [Reuters]

"In another sense, you talk to people in government who say that it could have been much worse - so there is definitely a sense of finger-pointing on who is to blame here.

"President Lula is demanding answers to how this happened and how to [prevent] it from happening again."

The blackout triggered a huge police mobilisation amid fears of an opportunistic crime wave.

Parts of neighbouring Paraguay, which also receives power from the Itaipu dam, were also affected. The dam straddles the border between Paraguay and Brazil.

Subway rail services were also knocked out in both Rio and Sao Paulo.

Edson Lobao, Brazil's mines and energy minister, said strong storms had uprooted trees near the dam just before it went offline and could be to blame for the sudden outage.

The blackouts came three days after a report on US TV network CBS said that several past power outages in Brazil had been caused by hackers attacking electricity control and distribution systems.