**Brazil’s two largest cities - Rio de Janeiro and San Paulo - are suffering major blackouts due to a problem at the Itaipu dam on the border with Paraguay.**Several other cities have also been affected by the power failures.
The country has lost 17,000 megawatts of power after the massive dam was hit by unspecified problems, local media reports.
The director of the Itaipu dam has said that it has lost its entire hydroelectric output.
The underground railway system in Rio de Janeiro shut down, after the power cuts hit at 2200 (0000GMT), leaving many passengers stranded.
No traffic lights or street lights are working in the city.
The power failture is also affecting cities in the northern part of the country, but not the capital, Brasilia.