New York: US Democratic presidential nominee Hillary Clinton's probability of winning the White House gained in online betting markets following the first debate of the campaign on Monday night between her and Republican Donald Trump.

A Clinton contract on the popular PredictIt betting market gained 6 cents from the previous day's level to 69 cents, while a contract favouring Donald Trump's prospects for victory tumbled 7 cents to 31 cents.

Contracts are priced from 0 cents to 100 cents, with the contract price equating to a probability of whether that candidate will win the November 8 election.

The price swings for both candidates were the largest since early August, and placed Clinton's lead in that market at the widest in about two weeks.