Trump's campaign has called for a recount in Wisconsin,where news organisations have pronounced Biden the winner by a razor-thin margin.
Protests have been scattered,small and largely peaceful since Americans went to the polls on Tuesday.
Facebook said it had taken down a rapidly growing group the social media site said pro-Trump activists had posted with violent rhetoric calling for"boots on the ground"to protect the integrity of the election.
Biden supporters have adopted the slogan"count every vote,"saying a complete and accurate tabulation in the remaining battleground states would show the former vice-president had won the 270 electoral votes needed to win.
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Both sides held rallies in Philadelphia on Thursday,where election staffers slowly counted thousands of mail-in ballots that could award Biden or Trump Pennsylvania's crucial 20 Electoral College votes.
Trump activists waved flags and carried signs saying:"Vote stops on Election Day"and"Sorry,polls are closed"as Biden supporters danced to music behind a barricade across the street.
"We can't allow the ballot counters to be intimidated,"said retired social worker Bob Posuney,a 70-year-old Biden supporter wearing a"count every vote"T-shirt as Marvin Gaye'sWhat's Going Onfilled the air.
In Harrisburg,about 100 Trump supporters gathered on the steps of the Pennsylvania State Capitol Building on Thursday afternoon as part of a"Stop the Steal"demonstration organised by Virginia conservative activist Scott Presler.
"This is not a rally supporting a particular candidate,"Presler said."This is a rally fighting for two things:truth and justice."He said in an interview that he planned to raise funds for an audit of the state's vote count.
Republican Jim Jordan of Ohio,an outspoken supporter of Trump,said at the state capitol building he was concerned about ballots arriving in the mail without a clear postmark.
Earlier on Thursday,a state appellate court ruled more Republican observers could enter the building in Philadelphia where poll workers were counting ballots.
At least 400 protesters gathered outside the Clark County Election Department in Las Vegas. Loud patriotic anthems blared over speakers as scores waved giant Trump and American flags. Biden has a lead of just over 11,000 votes in Nevada as counting continued.
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In Washington DC,a procession of cars and bicycles sponsored by activists from a group called Shutdown DC paraded slowly through the streets of the capital to protest what they called"an attack on the democratic process"by Trump and his"enablers,"according to its website.
Trump has said repeatedly without evidence that mail-in votes are prone to fraud,although election experts say that is rare in US elections.
-Reuters
Trump Biden 2020
Understand the election result and its aftermath with expert analysis from US correspondent Matthew Knott. Sign up toThe Sydney MorningHerald's newsletterhere,The Age'shere,Brisbane Times'here andWAtoday'shere.