Two people have died in an accident at Sao Paulo’s stadium, which is due to host the opening ceremony of the 2014 World Cup finals, police say.
A senior local fire official said crews were called to the Arena Corinthians after reports of a collapsed crane.
It fell on top of a metal structure, destroying part of a facade.
The arena was due to be completed at the end of December to meet a Fifa deadline. Brazil has admitted it is struggling to have all 12 venues ready.
The collapsed structure of the stadium can be seen from distance.
A small crowd of local residents has gathered outside to follow developments, but there are no workers around. One person told me it had felt like an earthquake.
There is a deep sense of anxiety among the security staff, as helicopters – some possibly from news outlets – fly over the almost-completed stadium.
Stadium supervisor Andres Sanchez and a representative of Odebrecht, the building company, paid tribute to the victims at a news conference in the canteen.
Mr Sanchez looked nervous. After all, workers were having their lunch in the same canteen when the crane collapsed.
There is a real sense here that Fifa and the Brazilian authorities now have a real challenge on their hands to get the organisation for the World Cup back on track.
Brazilian police said two people had been killed in Wednesday’s accident, after earlier reports quoted fire officials as saying that three had died.
The crane operator was taken to hospital, but was later released.
Odebrecht Infrastructure, the company building the stadium, told the BBC that the accident happened shortly before 13:00 local time (15:00 GMT).
It said “the crane that hoisted the last module of the structure of the metal roof collapsed causing the fall of the part of the circulation area of the east building”.
A LED panel outside the venue was also hit.
The company stressed that the structure of the stadium was not damaged.
Most of the workers are believed to have been away on their lunch breaks at the time.
TV footage later showed the huge metal structure buckled on top of the stadium, which is also known as Itaquerao.
Corinthians, the club that will move into the new stadium, said in a short statement that it “deeply regrets the accident”.
Sepp Blatter, president of Fifa – football’s world governing body – said he was “deeply saddened by the tragic deaths”.
Fifa Secretary General Jerome Valcke said: “Extremely shocked by the news from Sao Paulo. Our thoughts are with the families of the victims of this accident.”
In a statement, the world football governing body added that “the safety of workers is the top priority for Fifa” and Brazil’s government.
The venue is being built by Brazil’s Odebrecht company, which recently said the arena was about 94% ready.
The stadium is expected to seat nearly 50,000 people. However, 20,000 temporary seats are to be added for the World Cup opening match due to a Fifa request.
This is the third fatal accident in World Cup stadiums in Brazil, with two other construction workers killed over the past two years.
Six of Brazil’s stadiums – including Itaquerao – are not yet ready for the World Cup next summer.
Construction problems and public protests about the money being spent have delayed the preparations, the BBC’s Wyre Davies in Rio de Janeiro reports.
On Tuesday, Danny Jordaan, who was behind South Africa’s preparations for the 2010 World Cup, said Brazil might have to start round-the-clock construction work if it was to get things ready for the tournament.