Democratic presidential nominee Hillary Clinton (L) and Republican presidential nominee Donald Trump arrive before the second presidential debate at Washington University in St. Louis, Sunday, Oct 9, 2016. [Photo/IC]

US Republican presidential candidate Donald Trump said in the opening moments of a debate on Sunday that he was embarrassed by a video in which he made obscene comments about groping women without consent, but dismissed it as "locker room talk."

Trump, who is facing a party rebellion over the 2005 video that emerged on Friday, said former President Bill Clinton had done worse to women. "Mine are words and his are action," he said. He also accused Bill Clinton of going on the attack against women who had alleged sexual misconduct by her husband.

Clinton said Trump's comments showed he was unfit for the White House.

"He has said the video doesn't represent who he is but I think it's clear to anyone who heard it that it represents exactly who he is," Clinton said in response.

A flood of Republicans have withdrawn their support for Trump over a 2005 video that emerged on Friday showing the businessman, then a reality TV star, talking on an open microphone about groping women and trying to seduce a married woman.

The controversy has pitched Trump, 70, into the biggest crisis of his 16-month-old campaign and deepened fissures between him and establishment Republicans with only a month to go to the Nov. 8 election.