Joe Biden said on Friday that he had secured the delegates needed to clinch the Democratic nomination and face Donald Trump in November's US presidential election.

"Folks, tonight we secured the 1,991 delegates needed to win the Democratic nomination," the former vice president said on Twitter.

"I'm going to spend every day fighting to earn your vote so that, together, we can win the battle for the soul of this nation."

Biden passed the 1,991 threshold to secure his party's nomination as counting continued from Tuesday's round of Democratic primaries.

He had been the presumptive Democratic challenger since Vermont Senator Bernie Sanders dropped out of the race in April and endorsed his onetime rival's run at the White House.

Biden reached the threshold with the country wracked by protests over the death of African-American George Floyd at the hands of police.

Floyd's death has reignited long-felt anger over police killings of African-Americans and unleashed a nationwide wave of civil unrest unlike any seen in the US since Martin Luther King Jr's 1968 assassination.

"This is a difficult time in America's history. And Donald Trump's angry, divisive politics is no answer," Biden wrote in a post on Medium.

"The country is crying out for leadership. Leadership that can unite us. Leadership that can bring us together."

'Equal justice'

Biden's response to the protests has been in marked contrast to Trump, who threatened to deploy the military against American citizens.

In his first major public speech since going into isolation in mid-March because of the virus outbreak, Biden called Floyd's death a "wake-up call for our nation" and accused Trump of turning the US into a "battlefield riven by old resentments and fresh fears".

The 77-year-old Biden, who served as deputy for eight years to America's first black president, Barack Obama, has pledged to tackle "systemic racism" if elected to the White House.

"We need equal justice — and equal opportunities — for every American now. We need a president who cares about helping us heal — now," he wrote.

Biden's run for the Democratic nomination had looked destined for disaster following early losses to the fiery Sanders in Iowa, New Hampshire and Nevada.

But he came roaring back in South Carolina's primary in late February on the strength of overwhelming backing from African-American voters, a crucial base of Democratic support.

Biden will now be expected to name his running mate, after promising to pick a woman.

Senator Kamala Harris, a 55-year-old former attorney general of California, is considered one of the front-runners to be Biden's vice presidential pick.

Harris, who is of Jamaican and Indian ancestry, was considered a leading contender for the Democratic presidential nomination but dropped out in December after failing to break out of the crowded field.

In an average of opinion polls, RealClearPolitics gives Biden a 7.1 point lead over Trump in the election.