By Brenda Goh

RIO DE JANEIRO (Reuters) - Chinese web-users unleashed their fury on the social media accounts of Australian Olympic gold medallist Mack Horton, demanding he apologise for calling swimming rival Sun Yang a "drug cheat".

The controversy clouded Horton's Saturday triumph in the 400 metres freestyle final over Sun, the London 2012 champion, with the latter suggesting Horton had made the remarks to affect him. Sun won the silver medal.

"You have offended against the sensitive (feelings) of the Chinese people," said one of more than 300,000 comments left on Horton's latest Instagram post of him celebrating his win on the medal podium.

"Apologise to Sun Yang!", said another on Horton's Facebook page that was peppered with insults, derogatory pictures, many of which carried the hashtag "#apologisetosunyang".

Many of the comments recalled a 2013 incident when the Australian men's freestyle relay team were suspended for using a sedative banned by their national Olympic committee (AOC) in a bonding session before the London Games.

The Chinese swimming team has also demanded an apology from Horton, state news agency Xinhua reported.

"We have been noticing what has been said in the past two days by Horton, who launched a malicious personal attack (on Chinese swimmers)," it quoted Chinese swim team manager Xu Qi as saying.

"We think his inappropriate words greatly hurt the feelings between Chinese and Australian swimmers.

"It is proof of a lack of good manners and upbringing. We strongly demand an apology from this swimmer."

The 20-year-old Australian had acknowledged after their final that there were tensions between him and Sun after a splashing incident during a recent practice.

"I used the words 'drug cheat' because he tested positive," he said.

A spokesman for the AOC said Horton was entitled to express his point of view.

"He has spoken out in support of clean athletes. This is something he feels strongly about and good luck to him," the spokesman said in an email to Reuters.

In 2014, it emerged that Sun had secretly served a three-month suspension after he tested positive for a banned stimulant. He said at the time the stimulant was in medication to treat a heart issue and did not enhance his performance.

Sun, the first Chinese swimmer to win an Olympic gold medal, is very popular at home but has been the subject of multiple controversies. In 2013, he spent a week in jail for crashing a car when driving without a license.

He gained widespread sympathy at home on Saturday after breaking down in tears following his loss.

Chinese media carried pictures and videos of him crying and hugging a friend with the phrase "Sun Yang don't cry" trending across social media.

(Reporting by Brenda Goh; Editing by Rex Gowar/Greg Stutchbury)