Donald Trump has declared "I was wrong and I apologise" after being caught on tape making shockingly crude comments about a married woman he tried to seduce.

Yet even as he did so, the Republican US presidential hopeful claimed the astonishing recording was "nothing more than a distraction" and his words were not nearly as egregious as former president Bill Clinton's marital affairs.

"I've said some foolish things," he said in a taped apology posted on his Facebook page early on Saturday. "But there's a big difference between the words and actions of other people. Bill Clinton has actually abused women."

Turning to his Democratic rival Hillary Clinton, Mr Trump accused her of having "bullied, attacked, shamed and intimidated" her husband's "victims".

His 90-second statement capped a jarring day that threatened to sink his presidential campaign and sent Republicans into a panic with early voting well under way in several states and a little more than a month until election day.

On Friday afternoon, The Washington Post and NBC News released a 2005 video on which Mr Trump describes trying to have sex with a married woman. He also brags about women letting him kiss and grab them because he is famous.

"When you're a star they let you do it," Trump says. "You can do anything." He adds seconds later: "Grab them by the pussy. You can do anything."

Within hours, the shock of the video led to widespread condemnation from inside Mr Trump's own party. House of Representatives speaker Paul Ryan said he was sickened by the comments, while a one-sentence response from the party's chairman was devastating.

"No woman should ever be described in these terms or talked about in this manner. Ever," said Reince Priebus, who had stood by Mr Trump through his past provocative comments.

Mr Ryan added tartly that Mr Trump was "no longer attending" a joint campaign appearance set for Saturday in Wisconsin. Mr Trump himself later said he would be preparing for Sunday night's presidential TV debate instead.

Other Republicans, painfully aware of Mr Trump's possible impact on their own political fates, were quick to chime in. New Hampshire senator Kelly Ayotte, who is locked in a close race, called his comments "totally inappropriate and offensive".

By the time Mr Trump posted his video apology, three Republican members of Congress had called on him to abandon the race. Among them was Utah's Jason Chaffetz, who called Mr Trump's words "some of the most abhorrent and offensive comments that you can possibly imagine".

Mr Trump's running mate, Indiana governor Mike Pence, was "beside himself" and his wife was furious, according to a person familiar with their thinking.

On the tape, Mr Trump is caught on a live microphone while talking with Billy Bush of Access Hollywood. The candidate is heard saying "I did try and f*** her. She was married." He also uses graphic terms to describe the woman's body and says he frequently tries to kiss beautiful women.

Access Hollywood said a recent story about Mr Trump's lewd behind-the-scenes comments as star of The Apprentice led it to dig through its archives and turn up the previously unaired tape. It was recorded during a bus ride while Mr Trump was on his way to appear in an episode of the soap opera Days of Our Lives.

Mr Trump offered a half-hearted apology shortly after the video was released, saying he was sorry "if anyone was offended". Only hours later, after the scope of the damage became clear, did he release the video statement.

He appears alone in the video and appears to be reading off a script. He closes the video by suggesting he will raise Bill Clinton's affairs again in the coming days.

"See you at the debate," he says.

Mrs Clinton seized on Mr Trump's quotes from the 2005 video, calling them "horrific" and saying on Twitter: "We cannot allow this man to become president."

But she had her own problems on Friday with sudden revelations.

The WikiLeaks organisation posted what it said were thousands of emails from Clinton campaign chairman John Podesta, including some with excerpts from speeches she gave to Wall Street executives and others - speeches she has declined to release despite demands from Mr Trump.

The excerpts include Mrs Clinton seeming to put herself in the free trade camp, a position she has retreated from. In a talk to a Brazilian bank in 2013, she said her dream was "a hemispheric common market, with open trade and open borders".

Mr Trump strongly opposes current US trade deals and insists Mrs Clinton is too cosy with Wall Street to reform it.

Friday's developments came two days before Mr Trump and Mrs Clinton are to meet in the second presidential debate, with the Republican urgently in need of a strong performance.

After his uneven showing in the first contest, public opinion polls have showed Mrs Clinton pulling ahead in nearly all battleground states, some of which are already in the midst of early voting.