Brazilian lightweight Robson Conceicao claimed he was the victim of biased judging after he lost in the first round to Britain's Josh Taylor on Sunday.
Conceicao, who pushed world amateur champion Vasyl Lomachenko all the way in last year's championships, lost 13-9 but was convinced he won every round.
"They (the judges) were very malicious. It's not fair because I think the judges favoured him because of the crowd and that shouldn't happen in a competition like this," Conceicao told reporters.
"It hurts a lot, I was fighting really well, making the points and the referees didn't give it. I'm sad."
In front of a packed and giddily excited arena, Taylor became the third British boxer in a row to advance to the second round of this year's Games, handing rising boxing power Brazil its first defeat in the process.
"It will be very difficult," Conceicao said when asked if it would be hard to beat a British boxer in London.
Taylor, who sparred with the Brazilian in what he described a tough session in the run up to the Games, said it was amazing having the crowd behind every punch he threw, describing the experience as the best in his life.