Italy's Angela Carini abandoned her Olympic bout against Algerian boxer Imane Khelif inside 46 seconds, saying: "I had to preserve my life."
Khelif is one of two athletes who have been cleared to compete in women's boxing in Paris, having been disqualified from last year's Women's World Championships for failing to meet eligibility criteria.
The International Olympic Committee (IOC) - which did not organise the World Championships but is running the boxing at Paris 2024 - said welterweight Khelif was disqualified in India because of elevated levels of testosterone.
Having received a bye in the first round at the Olympics, the 25-year-old - who also competed at the Tokyo Games - entered Arena Paris Nord on Thursday to much applause from the many Algerians in the crowd.
After taking a punch to the face within 30 seconds, Carini went to the corner for her coach to fix her headgear but after briefly resuming, returned to her corner once more and stopped the fight.
Just before Khelif's arm was raised by the referee, Carini could be heard on camera saying "It's not right".
She was in tears in the ring and as she faced the media after the bout.
"I wasn't able to finish the match, I felt a strong pain in my nose and I said [to myself] for the experience that I have and the maturity as a woman that I have, I said I hope my nation won't take it badly, I hope my dad won't take it badly - but I stopped, I said stop for myself," Carini told BBC Sport.
"It could have been the match of a lifetime, but I had to preserve my life as well in that moment.
"I didn't have fear, I don't fear the ring. I don't fear taking the blows. But this time there's an end to everything, and I put an end to this match because I wasn't able to (continue)."
Italian Prime Minister Giorgia Meloni told Italian media: "It matters to be able to compete on equal grounds and from my point of view it was not an even contest."
But speaking about Khelif, Carini told reporters: "I wish her to carry on until the end and that she can be happy. "I am someone who doesn't judge anyone. I am not here to give judgements."
Khelif, who has lost nine times during her 50-fight career, told BBC Sport: "I'm here for the gold - I fight everybody."
The bout took place a day after Algeria's Olympic Committee condemned "baseless" attacks on Khelif.
Taiwan's Lin Yu-ting, who was stripped of a bronze medal at last year's World Championships after failing a gender eligibility test, fights in Paris on Friday.
The IOC said all boxers in Paris "comply with the competition's eligibility and entry regulations".
On Tuesday, IOC spokesman Mark Adams said: "These athletes have competed many times before for many years, they haven't just suddenly arrived - they competed in Tokyo."
The 2023 World Championships, at which Khelif and Lin were disqualified, were organised by the International Boxing Association (IBA).
But last June, the Russia-led body was stripped of its status as the sport's world governing body by the IOC, which organised the boxing competitions at the Tokyo 2020 Olympics and is doing so again for Paris.
On Wednesday, the IBA said the pair were disqualified at the World Championships "to uphold the level of fairness and utmost integrity of the competition".
It said they "did not undergo a testosterone examination but were subject to a separate and recognised test, whereby the specifics remain confidential".
"This test conclusively indicated that both athletes did not meet the required necessary eligibility criteria and were found to have competitive advantages over other female competitors," the body added.
The pair had tests at the World Championships in Istanbul in 2022 and then again in 2023.
The IBA said while Lin did not appeal against the disqualification decision at the Court of Arbitration for Sport, Khelif did appeal but later withdrew it.
Khelif will face Hungary’s Anna Luca Hamori in the 66kg quarter-finals on Saturday, where another win would guarantee her an Olympic medal.
Hamori told BBC World Service Sport: "My mentality is never to give up, it doesn't matter what happens.
"(Carini's abandonment] is her choice. I can promise I will fight to the end. We will see what happens.
"I don’t know what the truth is. I don’t care. I just want to win."
BBC Sport has contacted the Italian Olympic Committee and the IOC fo