Andres Sanchez, operations director at the Arena Corinthians site, told a press conference that the workers, aged 44 and 42, were crushed to death as a crane hoisting a 500-ton metallic piece to the top of the roof collapsed.
He said the cause of the mishap was not yet known but was under investigation.
He insisted that the overall structure was not affected.
The accident came amid a scramble to meet a December 31 set by football’s governing body FIFA to complete work at the venue.
A third worker was rushed to hospital with injuries after the accident, which brought a shocked response from FIFA president Sepp Blatter and condolences from the Brazilian ministry of sport.
“I’m deeply saddened by the tragic death of workers @Corinthians arena today,” Blatter said on Twitter. “Our heartfealt condolences are with the families.”
The toll could have been worse but luckily most of the workers were away from the site on a lunch break.
FIFA, which has faced recent scrutiny over worker conditions at under-construction venues for the 2022 World Cup in Qatar, later said safety standards for workers were “a top priority.”
It was not clear what impact the accident will have on the work but civil defence officials said work on 30 per cent of the site would be stopped.
Firefighters, ambulances and a police helicopter were at the scene, which was cordoned off.
Arena Corinthians is one of 12 World Cup venues planned for the tournament, for which Brazil has been struggling to overhaul its creaking infrastructure.
In addition to the opening of the Cup, the arena will host five other games, including the semi-final, while the final will be played in Rio’s iconic Maracana stadium next July 13.
The reported USD 11 billion cost of staging the event has aroused public protests.
One site worker, Jose Mario da Silva, 48, told the G1 news portal the accident happened as most workers were having a lunch break.