Great Britain’s Lutalo Muhammad suffered an agonising last-second defeat in the men’s -80kg taekwondo final - losing 8-6 to Cheick Sallah Cisse of the Ivory Coast.
World number four Muhammad, 25, led 6-4 but was hit by a reverse turning kick with the last action of the final.
GB were aiming for their second taekwondo gold of the Games after Jade Jones’ -57kg triumph on Friday.
"I’m so distraught," said a tearful Muhammad after his defeat.
"I was so close to becoming Olympic champion and making my dream.
"I don’t want to cry but I am so sorry to the people that stayed up to watch. I let them down at the last second. This is so hard.
"I was so close to being Olympic champion. The emotions are so raw. It’s hard to say how disappointed I am. It should have been gold. I am so upset I can’t speak clearly.
"I am absolutely gutted to blow it like that. I’ll have to wait four years for another chance. It’s one of the low points of my life."
Cisse won Ivory Coast’s first Olympic gold and just their third medal at any Games.
They had not won a medal of any colour since an athletics silver in Los Angeles in 1984 - but in addition to Cisse’s victory, Ruth Marie Christelle Gbagbi also won a bronze in the women’s -57kg in Rio.
Muhammad, a bronze medallist from London 2012, was patient at the start of his final before a kick to Cisse’s head towards the end of the first round gave him a 3-0 lead.
Cisse responded with a kick of his own in the second round, and a high kick in round three to keep pace with Muhammad.
In a tense final minute, Muhammad appeared to set to claim gold before the Ivorian snatched victory and charged around the arena in jubilation.
Cook suffers shock defeat
Earlier, Muhammad’s former GB team-mate Aaron Cook suffered a shock 14-2 loss to Wei-Ting Liu.
Muhammad was controversially selected ahead of Cook for London 2012, with Cook electing to fight for Moldova in 2015.
"I’m devastated," Cook said. "I made the switch from GB after the devastation of missing London 2012 and not feeling I had a way back. Moldova gave me a chance and had such high hopes."
Cook’s hopes of making the repechage ended when Liu was beaten in the quarter-finals by eventual bronze medallist Oussama Oueslati of Tunisia.
Oueslati beat American Steven Lopez to make the podium, while Azerbaijan’s Milad Beigi Harchegani, who Muhammad had defeated in his semi-final, took the other bronze with victory over Poland’s Piotr Pazinski.