RIO DE JANEIRO — Heavy rain, lightning and strong winds caused blackouts that left nearly a third of Brazilians — 60 million people — in the dark, officials said Wednesday as they scrambled to restore confidence in the country's infrastructure before soccer's 2014 World Cup and the 2016 Olympics.

The weather made transformers on a vital high-voltage transmission line short-circuit, Brazil's energy minister said. Two other transmission lines also went down as part of an automatic safety mechanism.

"The problem was exclusively with the transmission lines," Energy Minister Edison Lobao said.

The blackout cut electricity to 18 of Brazil's 26 states and left them without power for up to four hours Tuesday night. The federal district that includes the national capital of Brasilia was spared. About 7 million people also lost water service in Sao Paulo. All of Paraguay briefly lost power.

The massive Itaipu dam on the border with Paraguay — the world's second-largest hydroelectric power producer — was completely shut down for the first time in its 25-year history.

In Brazil's largest cities of Rio and Sao Paulo, people were trapped in elevators, stranded on commuter trains or stuck in sweltering apartments during unusually hot spring temperatures that have hit the 90s.

"I wonder how this could have happened and am worried about what it does to Brazil's image, especially with the World Cup and Olympics coming up," said Wesley Aragao, a 24-year-old sailor who waited out the blackout at his parents' house in northern Rio. "Nobody likes to be left in the dark."

The Brazilian Olympic Committee would not comment on Tuesday's blackout, but among the guarantees made to the International Olympic Committee is that Rio will be isolated from the nation's power system exactly to avoid problems like this. The city will have its own direct energy feed during the games.

Analysts said the blackout shows Brazil's lack of investment in the power system at a time when Latin America's largest economy is booming.