The accident could delay the delivery of the stadium in Sao Paulo by FIFA's December deadline to have all 12 venues ready. Work was immediately halted at the Itaquerao Stadium, which was practically completed before the collapse.
Television images showed a huge metal structure buckled atop the stadium, destroying part of the stands in the east side of the venue. A LED panel installed outside the venue also was hit.
Firefighter official Mauro Lopes said in a radio interview that at least three people died in the accident, which apparently happened when a crane collapsed on top of the metal structure. The crane was installing what was the last part of the structure.
Brazilian club Corinthians, which owns the stadium, released a brief statement "lamenting" the accident.
Constructor Odebrecht said it was already investigating what happened.
Brazil is running against time to deliver the last six World Cup stadiums by the end of the year, although work at the Itaquerao was advanced compared to the other venues.