The United States presidential race hangs in the balance despite an attempt by Donald Trump to claim victory and threaten legal action to stop votes in key states from being counted.
As the world watched on, Americans voted in record numbers to decide who would lead a divided country through a surging pandemic, an economic downturn, and social turmoil.
Trump, the Republican incumbent, led in most battleground states last night, but could not claim victory because of the unprecedented numbers of voters who posted their ballots rather than queue at crowded polling stations during an uncontrolled Covid-19 outbreak.
In a 2.30am address to supporters in the White House, he described counting of mail-in ballots as "fraud" and prematurely claimed he had won.
"This is a fraud on the American public," he said. "This is an embarrassment to our country."
He added: "We were getting ready to win this election. Frankly, we did win this election."
Trump made the comments as the key states of Michigan, Pennsylvania, Georgia, and Wisconsin remained too close to call and were waiting on large numbers of urban and mail-in ballots which were likely to favour his Democratic rival.
He promised to fight the election results at the Supreme Court, which has a 6-3 Republican majority.
"We will be going to the US Supreme Court. We want all voting to stop," Trump said.
At the same time, he criticised a television network for declaring Arizona for Biden and said votes were still being counted.
The large number of mail-in ballots and states' varied approaches to counting them meant the most powerful country in the world might not know its next leader for another day, or possibly longer.
It is the closest election race since George W Bush beat Al Gore in 2000. Bush won after a recount in Florida was controversially halted by the Supreme Court.
The mailed-in ballots - which take longer to process and in many states are not counted until election day - were expected to favour Biden. The Democratic challenger was behind last night in the battleground states of Georgia, Pennsylvania, Wisconsin and Michigan but led in Arizona.
"We feel good about where we are, we really do," Biden told supporters in his home state of Delaware last night.
"We believe we are on track to win this election. We knew because of the unprecedented mail-in vote; it was going to take a while. We're going to have to be patient.
"It ain't over until every vote is counted."
Most of Trump's supporters were expected to have voted on election day, while more than half of Biden's supporters cast their vote in the mail.
The United States' inability to immediately choose its next leader was mostly down to states' different approaches to counting mail-in ballots.
Florida and North Carolina were able to begin counting mailed votes weeks before election day.
Michigan, Pennsylvania and Wisconsin, on the other hand, made a decision not to count mail ballots before election day. All of these swing states have Republican-led legislatures.
Americans made their choices as the nation faced a confluence of historic crises with each candidate declaring the other fundamentally unfit to navigate the challenges. Daily life has been upended by the coronavirus, which has killed more than 232,000 Americans and cost millions of jobs.
Voters put aside worries about the virus — and some long lines — to turn out in person, joining 102 million fellow Americans who voted days or weeks earlier, a record number that represented 73 per cent of the total vote in the 2016 presidential election.
Biden entered election day with multiple paths to victory, while Trump, playing catch-up in several key states, had a narrower but still feasible road to clinch 270 Electoral College votes.
After the first polls closed, Biden picked up expected wins in Democratic-leaning states: Colorado, Connecticut, Delaware, Illinois, Maryland, Massachusetts, New Jersey, New Mexico, New York, Rhode Island, Vermont and Virginia.
Trump also claimed predictable victories in Alabama, Arkansas, Kentucky, Louisiana, Mississippi, Nebraska, North Dakota, Oklahoma, South Dakota, Tennessee, West Virginia, Wyoming, Indiana and South Carolina.
The most tense early battle was in Florida, where the vote see-sawed between the two presidential candidates before Trump pulled away - a lead credited to stronger than expected support among Hispanic voters.
The state was a crucial part of keeping Trump's re-election bid alive. Biden had other pathways to victory even if he lost Florida.
Texas, which has been Republican since 1976, leaned towards Biden in early counting but was eventually back in Trump's hands.
The two candidates earlier spent the day thanking supporters.
Trump predicted a "tremendous" night while visiting his campaign headquarters but also struck a note of caution: "Winning is easy, losing is never easy. Not for me, it's not."
Biden visited his childhood home in Scranton, Pennsylvania, and signed his name on the wall with the message: "From this house to the White House with the grace of God."
But for all the positivity of vast numbers of Americans carrying out their simple act of democracy, the country was braced last night for far more turbulent days ahead.
An 8ft-high "unscalable" fence had been erected during Monday night around the White House, similar to the ones put in place during heated anti-racism protests over the summer.
The scene was repeated in city downtowns across the US, with some National Guard units on standby. In Washington DC, police said that demonstrations were expected whatever the outcome.