US President Donald Trump has reaffirmed support for his Supreme Court nominee Brett Kavanaugh after the judge gave hours of testimony denying having sexually assaulted several women in his youth.

Earlier, a defiant Mr Kavanaugh angrily denied sexual assault allegations on Thursday and condemned his bitter Senate confirmation process as a “national disgrace.”

Kavanaugh’s passionate defense came after a university professor, Christine Blasey Ford, told the Senate Judiciary Committee that he sexually assaulted her 36 years ago.

The allegations against the 53-year-old conservative judge have thrust the Trump administration into the #MeToo movement’s harsh glare, and threaten to derail its bid to tilt the nation’s highest court to the right for years to come.

“I categorically and unequivocally deny the allegation by Dr. Ford,” said Kavanaugh, whose voice shook with anger during an opening statement that saw him repeatedly shed tears. “I’ve never sexually assaulted anyone, not in high school, not in college, not ever.”

Donald Trump has backed his nominee.

AP

“I am innocent of this charge,” Kavanaugh.

“Judge Kavanaugh showed America exactly why I nominated him. His testimony was powerful, honest, and riveting,” Mr Trump tweeted.

Like many Americans, the president spent much of the day watching the live broadcast of the hearing. The White House said he even watched a feed on Air Force One as he flew back from New York at the start of the Senate action.

Mr Trump had angered many women’s rights activists this week by seeming to suggest that the accusers must be making up their stories, because the alleged events dated to decades ago. Experts say that women often keep assault stories secret for long periods out of shame or fear.

However Trump softened his position on Wednesday, telling journalists that he was ready to change his mind, depending on the testimony.

“I can always be convinced,” he said. “It’s possible I’ll hear that, and I’ll say, ‘Hey, I’m changing my mind.'”

Earlier, during four hours of emotionally-charged testimony, Blasey Ford, 51, said she was “100 percent” certain Kavanaugh was her assailant and it was “absolutely not” a case of mistaken identify.

“I am here today not because I want to be,” Blasey Ford said as she recounted the sexual assault which she alleged occurred at a high school party at a suburban Maryland home in 1982.

Christine Blasey Ford is sworn in before testifying the Senate Judiciary Committee.

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“I am terrified,” she said, her voice often quavering. “I am here because I believe it is my civic duty to tell you what happened to me.”

Kavanaugh condemns ‘character assassination’

Kavanaugh slammed what he called a “grotesque and coordinated character assassination” and a “calculated and orchestrated political hit.”

“My family and my name have been permanently destroyed by vicious and false accusations,” Kavanaugh said.

“This confirmation process has become a national disgrace.”

But he said he would not withdraw his candidacy for a spot on the nine-member Supreme Court.

“I will not be intimidated into withdrawing from this process,” Kavanaugh said. “You may defeat me in the final vote but you’ll never get me to quit. Never.”

Brett Kavanaugh is in a tumultuous confirmation process as multiple women have accused him of sexual misconduct.

AAP

‘I believed he was going to rape me’

Earlier, during four hours of emotionally-charged testimony, Blasey Ford, 51, said she was “100 percent” certain Kavanaugh was her assailant and it was “absolutely not” a case of mistaken identify.

Blasey Ford said she thought she was going to be raped. 

“He began running his hands over my body and grinding into me,” she said. “I yelled, hoping that someone downstairs might hear me.

“Brett groped me and tried to take off my clothes,” she said. “He had a hard time because he was very inebriated.

“I believed he was going to rape me,” Blasey Ford said. “I tried to yell for help.

“When I did, Brett put his hand over my mouth to stop me from yelling,” she said. “This is what terrified me the most and has had the most lasting impact on my life.

“It was hard for me to breathe, and I thought that Brett was accidentally going to kill me,” she said.

“Indelible in the hippocampus is the laughter, the uproarious laughter between the two, and them having fun at my expense,” she said when asked about her most powerful memory of the incident.

Dr. Christine Blasey Ford speaks before the Senate Judiciary Committee hearing.

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Blasey Ford said she managed to escape when Judge jumped on the bed, sending them all toppling.

Outside the US Capitol, pro- and anti-Kavanaugh demonstrators gathered as senators pondered the fate of a nominee who could impact the balance of the top US court for decades.

Kavanaugh slammed what he called a “grotesque and coordinated character assassination” and a “calculated and orchestrated political hit.”

“My family and my name have been permanently destroyed by vicious and false accusations,” Kavanaugh said.

“This confirmation process has become a national disgrace.”

Trump says he may consider withdrawing his support

Trump nominated Kavanaugh to replace Justice Anthony Kennedy, who had been a swing vote on a court currently divided between four conservative and four liberal justices.

The Republican president has fiercely defended his pick as a “great intellect” – but said Wednesday he would watch the highly charged hearing and was open to changing his mind.

Brett Kavanaugh is sworn in to testify before the Senate Judiciary Committee hearing.

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“I can always be convinced,” Trump said.

At the same time, Trump said he was skeptical because he personally had been the target of “false charges” by various women.

The White House confirmed that Trump watched the testimony aboard Air Force One as he returned to Washington from New York.