After days of nail-biting suspense following the Nov 3 US presidential election, Democratic Party candidate Joe Biden marked his victory with a call for unity, urging Americans to "come together as a nation" after one of the most fractious election campaigns in recent US history.

"Let this grim era of demonization in America begin to end here and now," Biden told cheering supporters at a flag-bedecked victory party in his hometown of Wilmington, Delaware, on Saturday night.

Biden also said he would unveil a group of top scientists on Monday to help him craft a plan to tackle the coronavirus pandemic, which had claimed the lives of at least 237,000 Americans as of Saturday evening, according to Johns Hopkins University data.

Biden's triumph made Republican President Donald Trump the first US president since 1992 to fail to win a second term. In addition, Biden's running mate Kamala Harris will become the first US woman to serve as vice-president, as well as the first black and South Asian-American woman to shatter a significant glass ceiling.

Biden was declared to have defeated Trump on Saturday morning, when major US media outlets, including the Associated Press, CNN, CBS and Fox News, called the key battleground state of Pennsylvania for the 77-year-old.

By Saturday evening, Biden had taken at least 279 electoral votes against Trump, according to multiple US media projections.

Other forecasts have a higher number of electoral votes for Biden, far over the threshold of 270 Electoral College votes he needs to clinch the presidency.

"With the campaign over, it's time to put the anger and the harsh rhetoric behind us and come together as a nation," Biden said in a statement earlier in the day. "It's time for America to unite. And to heal."

The president-elect, however, is likely to experience a potentially turbulent transfer of power, as Trump is refusing to concede defeat, and his campaign has indicated that it will challenge election outcomes in a handful of states with lawsuits.

The sitting president immediately accused Biden of "rushing to falsely pose as the winner" as media outlets made their calls for his rival.

"This election is far from over," Trump said in a statement.

"Joe Biden has not been certified as the winner of any states, let alone any of the highly contested states headed for mandatory recounts, or states where our campaign has valid and legitimate legal challenges that could determine the ultimate victor," he said.

William C. Banks, distinguished professor emeritus at Syracuse University College of Law in New York, said it is highly unlikely that Trump can do anything to change the outcome of the election.

Across the country, there were celebrations as the news broke that the presidential race had been called for Biden.

In cities including New York, Washington and Atlanta, residents took to the streets with spontaneous block parties. People also gathered at the Black Lives Matter Plaza near the White House, waving signs and taking cellphone photos.

Trump supporters, chanting "This isn't over!" and "Stop the steal", protested at state capitals across the US on Saturday, refusing to accept defeat and echoing Trump's unsubstantiated allegations that the Democrats won the election by fraud, the Associated Press reported.

"It remains to be seen how his supporters react," Banks said. "It is too soon to tell."

Cal Jillson, a political scientist and historian at Southern Methodist University in Dallas, said there was "tremendous energy" in this election on both sides, as it appears that Bidenmay have won 80 million votes by the time that it is all over, possibly 10 million more than the previous record, and Trump may win 75 million votes.

Trump's conduct out of office over the coming years may keep his supporters stirred up, but that is probably not as important as whether the Senate Republican majority leader Mitch McConnell chooses to look for common ground with Biden or resist his early initiatives, according to Jillson.

Races for control of the US Senate have yet to end. If the Republicans retain the majority, they would likely make it difficult for the new administration to carry through its legislative agenda, including expanding healthcare, fighting climate change and raising taxes on the wealthy, according to US media analysis.

Many world leaders were quick to offer their congratulations to Biden.

French President Emmanuel Macron, in a message on social media, said, "The Americans have chosen their president."

"We have a lot to do to overcome today's challenges. Let's work together," he added.

British Prime Minister Boris Johnson also congratulated Biden and the "historic achievement" of Harris' election as vice-president.

Other leaders who sent congratulatory messages included Canadian Prime Minister Justin Trudeau, German Chancellor Angela Merkel, NATO Secretary General Jens Stoltenberg, Egyptian President Abdel-Fattah el-Sissi, Indian Prime Minister Narendra Modi, Japanese Prime Minister Yoshihide Suga and Iraqi President Barham Salih.