The US Senate confirmed President Donald Trump's Supreme Court pick Brett Kavanaugh on Saturday in the closest such vote in more than a century, amid controversy over sexual abuse allegations against him.

The Senate voted 50-48 to approve Kavanaugh in a major win for Trump as more than 1,000 protesters rallied in Washington against a nominee who had to overcome questions over his candour, partisan rhetoric and lifestyle as a young man.

The months-long battle over the nomination of Kavanaugh, who has weathered accusations of sexual assault, has roiled Americans' passions.

The Senate vote brings an end to a raucous nomination process defined by harrowing testimony from a woman who says Kavanaugh tried to rape her when they were teenagers — and by his fiery rebuttal.

With Kavanaugh's confirmation, Trump has succeeded in having his two picks seated on the court — tilting it decidedly to the right in a major coup for the Republican leader less than halfway through his term.

In a sense it reflects a high water mark of the Trump presidency: Republican control of the White House, the Senate, the House of Representatives and the judiciary's top court.

But the Kavanaugh spectacle, fuelled by extraordinary accusations and counter-claims in nationally televised hearings, and tense battles over an 11th-hour FBI investigation to address the assault allegations, has enflamed political passions.

'Shame!'

With just hours before the vote, more than 1,000 protesters, mostly women, broke through barricades and staged a raucous sit-in protest on the US Capitol steps, just feet away from the imposing doors to the Rotunda.

As protesters chanted “Shame!” and “November is coming!” police took several dozen protesters down the steps and put them in plastic flex-cuffs.

Kavanaugh's confirmation process has laid bare the partisan gridlock on Capitol Hill and the political polarisation of America just a month before midterm elections.

His promotion to the Supreme Court also stands as a demoralising defeat for Democrats who battled hard to block the 53-year-old judge at all costs.

But Senator Ed Markey insisted the drama playing out will only galvanise Democrats to deliver a “devastating” blow to Republicans at the ballot box.

“The Democrats are going to pivot to the election, and we're going to turn this nomination into a referendum on whether or not Donald Trump can be trusted to name federal judges or to continue to control an absolute monopoly on creating public policy in the United States,” Markey told reporters.

Kavanaugh's confirmation was all but sealed on Friday when he won the support of key Senate Republican Susan Collins and conservative Democrat Joe Manchin.

Their announcements brought the number of senators supporting Kavanaugh to 51 in the 100-member chamber.

“This is a great day for America,” Senate Majority Leader Mitch McConnell told Fox News late Friday, congratulating his colleagues for “refusing to roll over under all of this intense pressure”.

'Presumption of innocence'

Kavanaugh's nomination as a replacement for retiring justice Anthony Kennedy was controversial from the start — but the initial focus was solely on the conservative views held by the married father of two.

His ascent to the Supreme Court was thrown into doubt last week after university research psychologist Christine Blasey Ford testified that he had sexually assaulted her at a Washington area party in the early 1980s.

The brutal hearing sparked a supplemental FBI dive into Kavanaugh's background and a week-long delay of the Senate vote.

While many senators say they were satisfied with the FBI probe, her lawyers say the investigation was insufficient.

“An FBI investigation that did not include interviews of Dr Ford and Judge Kavanaugh is not a meaningful investigation in any sense of the word,” they said in a statement quoted in US media.

Collins — a moderate Republican from Maine — said Kavanaugh was entitled to the “presumption of innocence” as the allegations against him were not substantiated with corroborating evidence.

While acknowledging that Blasey Ford's testimony was “sincere, painful and compelling”, Collins added: “We will be ill-served in the long run if we abandon the presumption of innocence and fairness.”

Immediately after that speech, Manchin announced his support, calling Kavanaugh a “qualified jurist” who “will not allow the partisan nature this process took to follow him onto the court”. Manchin faces extraordinary political pressure. He is up for re-election in West Virginia, a state Trump won overwhelmingly in 2016.

The stage was set for Saturday's final confirmation when the Senate ended debate on the nomination on Friday with a procedural 51-49 vote — a move cheered by Trump, who said he was “very proud.”

'Agonising'

Kavanaugh's confirmation win seals a conservative majority on the nine-seat high court, possibly for decades to come.

His nomination has been met with loud protests, both in Washington and in other cities across the United States. Hundreds have been arrested on Capitol Hill this week.

Authorities took the rare step of putting up low metal fences around the Capitol, keeping the public some distance from the building. But protesters overran the barricades and defiantly claimed the Capitol steps.

Trump tweeted that the demonstrators included female Kavanaugh supporters who were “gathering all over Capitol Hill”. But they were not immediately visible in the area around the legislature.

The president claimed on Friday that billionaire financier George Soros, a Democratic funder and frequent target of conservatives, was behind the demonstrations against his nominee.

Alaska Senator Lisa Murkowski was the only Republican to break ranks in Friday's cloture vote.