NEW YORK (REUTERS) - Republican presidential candidate Donald Trump stole the social media spotlight during a US presidential debate on Monday night (Tuesday morning Singapore time) - this time for what Twitter users branded a #Trumpsniffle.

The wealthy businessman was seen and heard sniffling repeatedly as he faced off against Democratic rival Hillary Clinton in their first debate, giving rise to the hashtag.

The Twittersphere was abuzz with playful memes and animations as viewers tried to figure out what was causing Mr Trump's nose to run. Parody accounts Donald's Sinuses (@TrumpsSinuses) and Trump sniff (@TrumpSniff) instantly gained a large following.

A campaign aide said Mr Trump, 70, did not have a cold.

Several tweeters seized on the sniffling to hit back at Mr Trump over his repeated digs at the health and stamina of Mrs Clinton, 68, who had pneumonia earlier this month. "I am worried about @realDonaldTrump's health - are the sniffles symptoms of something more serious? #sniff #debatenight," tweeted user Scott Charton.

Twitter said the debate was the most tweeted-about political moment in the social media company's history. Mr Trump was the focus of 62 per cent of the conversation on the social media platform, Twitter said.

On Facebook, conversations about Mr Trump made up 79 per cent of debate chatter, while Mrs Clinton's share of the conversation was 21 per cent.

Even so, sentiment appeared to go Mrs Clinton's way. Social media analytics firm Zoomph said tweets mentioning Mrs Clinton ended at a ratio of about 1.5 to 1, which meant that for every negative mention, there were 1.5 positive mentions, Zoomph said.

Sentiment towards Mr Trump fluctuated, but ended nearly flat at a ratio of one positive mention to every negative one.

The most tweeted-about topics were the economy, foreign affairs, energy and environment, terrorism and guns.