President Donald Trump says he will take the US election results to the Supreme Court, claiming a "fraud on the American nation" had been committed and declaring "we did win".
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Despite incomplete results from battleground states that could determine the outcome of the race, Trump early on Wednesday, US time, proclaimed victory over Democratic challenger Joe Biden.
"We are up BIG, but they are trying to STEAL the Election. We will never let them do it. Votes cannot be cast after the Polls are closed!" Trump tweeted before his White House appearance, where he went on to tell supporters, "Frankly, we did win".
But election results from some battleground states, including Pennsylvania, Wisconsin, Michigan and Georgia, were still not clear and projections from major networks and Edison Research showed Trump still short of the 270 electoral votes need to win re-election.
Trump said he wanted all voting to stop and no more votes found to count. Several states allow mailed-in votes to be accepted after Election Day, as long as they were postmarked by Tuesday. That includes Pennsylvania, where ballots can be accepted if they arrive up to three days after the election.
Trump suggested those ballots shouldn't be counted. But Biden, briefly appearing in front of supporters in Delaware, urged patience, saying the election "ain't over until every vote is counted, every ballot is counted."
Mr Trump's claim brought immediate condemnation and came shortly after Biden had told supporters he was on a path to victory but has warned that the results would not be known for at least another day.
"Keep the faith guys, we're gonna win this," Biden told a crowd outside the Chase Center in Wilmington, Delaware on Tuesday night.
"We believe we're on track to win this election," Biden said. "It ain't over until every ballot is counted... but we're feeling good about where we are."
Biden's confidence was soon bolstered by him picking up Arizona and its 11 electoral votes, flipping a critical battleground state that Donald Trump won four years ago and that could help determine which candidate wins the presidency.
The victory by Biden was a huge blow to Trump's chances for re-election. Arizona has backed a Democratic presidential candidate only once in the last 72 years. Speaking to honking supporters at a drive-in event outside the convention centre, the former vice president said the results would be clear as soon as Wednesday morning, but could take longer.
"It's not my place or Donald Trump's place to declare who won the election," he said. "That's the decision of the American people."
Trump was declared the winner of Florida shortly before Biden took the stage, taking that state's hotly contested 29 electoral votes.
Trump held a narrow margin in Georgia and North Carolina, while Biden looked to be clinching Arizona.
Biden's supporters were banking on reclaiming the three "Blue Wall" states that Trump won in 2016: Wisconsin, Michigan and Pennsylvania.
Biden's campaign had focused on Arizona as part of its expanded battleground map through the Sun Belt, citing demographic changes, new residents and realignment away from Republicans among key suburban voters.
Early results in several key battleground states were in flux as election officials processed a historically large number of mail-in votes.
Democrats typically outperform Republicans in mail voting, while the GOP looks to make up ground in Election Day turnout.
That means the early margins between the candidates could be influenced by which type of votes - early or Election Day - were being reported by the states.