Chinese swimming officials have demanded an apology after Australian Olympic gold medallist Mack Horton called defending champion Sun Yang a "drug cheat".
Tension between the swimming rivals had been simmering in the lead up to the men's 400m freestyle final in Rio.
Horton said earlier this week he had no "time or respect for drug cheats".
Sun served a three-month suspension in 2014 for testing positive for a banned substance but was cleared to compete.
His fans have also been posting a wave of abusive comments to Horton's social media accounts.
'Good luck to him'
Horton made a stunning Olympic debut in the 400m freestyle on Saturday, narrowly edging out Sun to win the race in a personal best time of three minutes, 41.55 seconds.
Before the race, Horton had accused Yang of deliberately splashing him in a training session, saying: "I ignored him, I don't have time or respect for drug cheats."
In an interview after his win he defended his comments, saying: "I used the words drug cheat because he tested positive.
"I just have a problem with athletes who have tested positive and are still competing."
The Australian Olympic Committee said Horton was "entitled to express a point of view", adding: "He has spoken out in support of clean athletes. This is something he feels strongly about and good luck to him."
But China's swimming team manager Xu Qi said it had been "a malicious personal attack".
"We think his inappropriate words greatly hurt the feelings between Chinese and Australian swimmers," he said, in comments carried by China's Xinhua news agency.
"It is proof of a lack of good manners and upbringing. We strongly demand an apology from this swimmer."
Footage of a devastated Sun bursting into tears after the race went viral on social media.
"Sun Yang Don't Cry" was the top-trending hashtag on Weibo, China's equivalent of Twitter.
Tens of thousands of people have since posted angry comments on Horton's social media.
"Horton is a loser because of his bad behaviour. Maybe he won gold in the game but he will never win in life," said one person on Twitter.
"Ur so disrespectful and RACIST to your competitor! You didn't even know the truth! Sun has heart issue. #ApologizeToSunYang" said another tweet.
On Facebook, one person wrote: "You're the worst you don't deserve the gold！Just 'cause an Asian is good at swimming doesn't make him a drug cheat. THAT'S RACIST".
"Do you know what it's called respect! You are a loser! Chinese people always look down on you!" said another.
In a case of mistaken identity, an Englishman named Mark Horton was abused on social media despite making it clear that he was not the Australian swimmer.
Sun was the first Chinese man to win an Olympic gold medal in swimming - winning the 400m and 1500m freestyle at the 2012 Olympics.
In 2014 it emerged that he had been banned for three months after testing positive for a banned stimulant trimetazidine, which he said had been for a heart complaint.
He dismissed Horton's comments, saying: "I've proved I'm clean. I don't think we need to concern ourselves with the Australian's mind tricks."
When asked if he would beat Horton in their next event, the world-record holder said: "In 1500m I am the king."