Protests heat up as counting drags on in cliffhanger US election

Backers of President Donald Trump ramped up demonstrations on Thursday night against an election they believe was rigged or being stolen, as vote counting continued in battleground states.

In Arizona, one of five US battleground states where votes were still being counted in the too-close-to-call race between Trump and Democratic challenger Joe Biden, Trump supporters massed outside the Maricopa County Elections Department in Phoenix.

Protests have been scattered, small and largely peaceful since Americans went to the polls on Tuesday.

Both sides held rallies in Philadelphia on Thursday, where election staffers slowly counted thousands of mail-in ballots that could decide 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.

Late on Thursday, Philadelphia police said they were investigating an alleged plot to attack the city's Pennsylvania Convention Center, where votes were being counted. Police took at least one man into custody and seized a weapon. No injuries were reported.

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.

Earlier on Thursday, a state appellate court ruled more Republican observers could enter the building in Philadelphia where poll workers were counting ballots.

In Milwaukee, some 50 Trump supporters gathered in front of a city building where votes were being counted, blasting country music, waving flags and carrying signs reading "Recount" and "Rigged".

Roughly a dozen counter-protesters arrived after an hour, shouting "Black lives matter" and "say their names," referring to the victims of police brutality. Others threw eggs at the Trump supporters from a passing car.

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.

In Washington, D.C., 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.

Facebook Inc 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.

Trump has said repeatedly without evidence that mail-in votes are prone to fraud, although election experts say that is rare in US elections.