Joe Biden has won the US presidency according to most major television networks, as voters narrowly ended Donald Trump’s tumultuous leadership.

Mr Biden acknowledged the result in a tweet early Sunday morning (Australian time), telling Americans he was “honoured that you have chosen me to lead our great country”.

“The work ahead of us will be hard, but I promise you this: I will be a President for all Americans — whether you voted for me or not. I will keep the faith that you have placed in me,” he said.

Mr Biden made no mention of Mr Trump, who becomes the first one-term president since George HW Bush in 1993.

Kamala Harris, 56, makes history as not only the first female Vice President of the United States, but the first black woman elected to the office.

When Mr Biden, the former vice president in the Obama administration, enters the White House on January 20, he will be the oldest person to assume the office at age 78.

https://twitter.com/JoeBiden/status/1325118992785223682

The President-elect is scheduled to make a public address at midday Australian time.

Mr Biden’s projected victory came after four days of nail-biting suspense over the outcome of Tuesday’s election, with the counting of votes in a handful of battleground states ongoing thanks to a flood of mail-in ballots.

Mr Biden said on Saturday he expected to win the race, but stopped short of giving a victory speech.

Less than an hour before Mr Biden was declared the winner, Mr Trump tweeted “I WON THIS ELECTION, BY A LOT!”. His post was immediately accompanied by a Twitter warning that it may not be true.

I WON THIS ELECTION, BY A LOT!

— Donald J. Trump (@realDonaldTrump) November 7, 2020

Mr Trump left the White House earlier in the day to attend his Virginia golf resort.

Mr Biden had a lead of 273 to 214 in the state-by-state Electoral College vote that determines the winner, having won Pennsylvania’s 20 electoral votes to put him over the 270 he needed to secure the presidency, according to Edison Research.

Cheering crowds took to the streets of cities across the United States at the news of the result.

The crowds already gathered in Washington quickly swelled with celebrating Biden supporters.

It’s starting to make sense why Trump left the White House this morning. pic.twitter.com/ol3MKQU2co

— Abdallah Fayyad (@abdallah_fayyad) November 7, 2020

World leaders have lined up to offer their congratulations, including British Prime Minister Boris Johnson, German Chancellor Angela Merkel, French president Emmanuel Macron, Canadian Prime Minister Justin Trudeau and New Zealand Prime Minister Jacinda Ardern.

“Congratulations to Joe Biden on his election as President of the United States and to Kamala Harris on her historic achievement,” Mr Johnson said in a statement.

“The US is our most important ally and I look forward to working closely together on our shared priorities, from climate change to trade and security.”

Former Australian Prime Minister Malcolm Turnbull described the Biden-Harris win as a “relief”.

Congratulations Joe Biden and Kamala Harris! What a relief that you won. 🙏🙏🙏

— Malcolm Turnbull (@TurnbullMalcolm) November 7, 2020

Former US presidents Barack Obama and Bill Clinton were also quick to congratulate their fellow Democrat, with Mr Obama declaring he “could not be prouder”.

To secure the win, Mr Biden faced unprecedented challenges including Republican-led efforts to limit mail-in voting at a time when a record number of people were due to vote by mail because of the pandemic, which has killed more than 235,000 people in the United States.

Both sides characterised the 2020 election as one of the most crucial in US history, as important as votes during the 1860s Civil War and the 1930s Great Depression.

For months, officials on both sides raised the spectre of the United States not being able to pull off a fair vote.

In the end, however, voting at the polls proceeded with limited disruption as millions lined up patiently to cast their ballot.

As world leaders quickly moved to congratulate Mr Biden, former president Barack Obama expressed pride in his former vice president.

In a statement, Mr Obama said Mr Biden has “got what it takes to be President and already carries himself that way”, because he will enter the White House facing “a series of extraordinary challenges no incoming President ever has”.

“I know he’ll do the job with the best interests of every American at heart, whether or not he had their vote,” he said.

Thousands of election monitors from both parties worked for four days to ensure the votes were being counted.

The election drama is likely to play out for weeks, if not months.

Mr Trump, 74, is contesting the vote in the courts, but legal experts said his challenges had little chance of affecting the outcome.

Mr Trump was reportedly playing golf when the result was announced. Photo: Getty

Mr Biden’s victory was driven by strong support from groups including women, African Americans, white voters with college degrees and city-dwellers.

He was more than four million votes ahead of Mr Trump in the nationwide popular vote count.

Mr Biden, who has spent half a century in public life as a US senator and then vice president under Mr Trump’s predecessor Barack Obama, will inherit a nation in turmoil over the coronavirus pandemic and the related economic slowdown as well as disruptive protests against racism and police brutality.

Mr Biden has said his first priority will be developing a plan to contain and recover from the pandemic, promising to improve access to testing and, unlike Mr Trump, to heed the advice of leading public health officials and scientists.

Mr Biden also has pledged to restore a sense of normalcy to the White House after a presidency in which Mr Trump praised authoritarian foreign leaders, disdained longstanding global alliances, refused to disavow white supremacists and cast doubt on the legitimacy of the US election system.

Despite his victory, Mr Biden will have failed to deliver the sweeping repudiation to Mr Trump that Democrats had hoped for, reflecting the deep support the president enjoys despite his tumultuous four years in office.

-with AAP