Joe Biden has said it is “time to heal” America in his first speech as president-elect, vowing “not to divide but unify” the country.
“Let’s give each other a chance,” Mr Biden said, addressing those who voted against him.
Mr Biden defeated incumbent President Donald Trump following a cliff-hanger vote count after Tuesday’s election.
Mr Trump has yet to concede and has not spoken publicly since his defeat was announced while he was playing golf.
The result makes Mr Trump the first one-term president since the 1990s. His campaign has filed a barrage of lawsuits in various states but election officials say there is no evidence that the vote was rigged against him.
Spontaneous celebrations erupted in major cities after media outlets announced Mr Biden’s victory on Saturday. Trump supporters angry at the result demonstrated in some cities but there were no reports of incidents.
Biden: ‘We have to stop treating our opponents as enemies’
Addressing cheering supporters in a parking lot in his hometown of Wilmington, Delaware, Mr Biden said: “I pledge to be a president who seeks not to divide, but to unify; who doesn’t see red states and blue states, only sees the United States.”
Mr Biden – who has won more than 74 million votes so far, the most ever for a US presidential candidate – hailed the “diverse” support he gathered during the campaign, but also reached out to Trump supporters directly.
“It’s time to put away the harsh rhetoric, lower the temperature, see each other again, listen to each other again,” Mr Biden said. “And to make progress, we have to stop treating our opponents as enemies.”
The president-elect, who arrived on stage wearing a face mask, announced that he would form his coronavirus response committee to ensure it is ready to implement decisions from his inauguration day in January.
The Trump administration’s response to the pandemic was at the centre of the presidential campaign, and drew heavy criticism from Mr Biden. More than 237,000 Americans have died, more than any other country in the world.
Mr Biden will be 78 when he takes office, making him the oldest first-term president in American history.
Harris: ‘You chose hope and unity’
Mr Biden was introduced by his running mate, Kamala Harris, who has made history as the first female, first black and first Asian-American US vice-president-elect.
“When our very democracy was on the ballot in this election, the very soul of America at stake and with the whole world watching, you ushered in a new day for America,” she said.
“You chose hope and unity, decency, science and yes, truth – you chose Joe Biden as the next president of the United States. And the road ahead will not be easy but America is ready, and so are Joe and I.”
She also noted the historic moment that the election result marked: “While I may be the first woman in this office, I will not be the last.”
Trump ‘not planning to concede’
The BBC’s projection of Mr Biden’s victory is based on the unofficial results from states that have already finished counting their votes, and the expected results from states like Wisconsin where the count is continuing.
It is projected that Mr Biden won the key battlegrounds of Pennsylvania and Nevada, propelling him over the 270 electoral college vote threshold required to clinch the White House.
Mr Trump has not yet spoken in public since the numbers were announced, but he repeated previous claims of voter fraud in a tweet, which Twitter soon marked as a “disputed” claim. The Trump campaign has indicated their candidate does not plan to concede.
After Mr Biden was projected to win Mr Trump remained defiant, saying Mr Biden was “falsely posing as the winner” and insisting the election was “far from over”. The president has drawn more than 70 million votes, the second-highest tally in history.
The response from senior Republicans has been muted. Republican Party Chairwoman Ronna McDaniel tweeted that “the media doesn’t decide who wins elections, voters do”.
But Senator Mitt Romney, a critic of Mr Trump, congratulated Mr Biden and Ms Harris. He said he and his wife “know both of them as people of good will and admirable character”.
Mr Trump has vowed to contest the election results on several fronts. A recount will be held in Georgia, where the margins are tight, and Mr Trump wants the same in Wisconsin. He has also vowed to take legal action to the Supreme Court, alleging voting fraud without evidence.
If the election result is challenged, it would require legal teams to challenge this in the state courts. State judges would then need to uphold the challenge and order a recount, and Supreme Court justices could then be asked to overturn a ruling.
Meanwhile, votes in some states are continuing to be counted and results are never official until final certification, which occurs in each state in the weeks following the election.
This must be done before 538 chosen officials (electors) from the Electoral College – which officially decides who wins the election – meet in their state capitals to vote on 14 December.
The electors’ votes usually mirror the popular vote in each state. However, in some states this is not a formal requirement.