Julia Carneiro, a BBC reporter in Rio de Janeiro, captured the incident on video outside the stadium, prior to Wednesday’s women’s football match between Sweden and South Africa.
Two men described as ‘firemen’ are seen using a pair of bolt cutters to break the padlock and open the gate.
“What a start! #Rio2016 kicking off with lost key and firemen forcing open lock to east gate of Olympic stadium #oops,” Carneiro wrote.
In a tweet sent one hour earlier, Carneiro claimed that spectators were forced to queue for up to two hours before being ushered to another entrance.
The incident, coming just two days before the opening ceremony, will embarrass Olympic organisers at a time of great scrutiny.
Earlier this week, it was revealed that some 1,400 aquatic athletes were at risk of becoming violently ill due to high levels of pollution in Rio’s waters.
Swimmers and other athletes who ingest just three teaspoons of water will almost certainly contract viruses that can cause stomach and respiratory illnesses.
1.3m tickets for the Games also remain unsold, with demand lower than in previous summer Games, though around 500,000 of those tickets are for football matches based outside of Rio.