Video posted to social media shows at least two dozen white men - most of them wearing bulletproof vests - encircling the blazing sign, hours after thousands of Donald Trump supporters attended a “Stop The Steal” rally against the election results.
One member of the group poured a flammable substance onto the fire, sending flames and thick black smoke billowing into the night sky.
Written underneath the words “Black Lives Matter” on the flag was the website of the Asbury United Methodist Church, a majority-Black but mixed congregation founded in 1836 and located a few blocks from the White House.
The Proud Boys group chanted "F*** Antifa!" referencing far-left anti-fascism protesters who they have repeatedly clashed with during racial justice protests this year following the police killings of Black Americans including George Floyd and Breonna Taylor.
Several of the protesters also made the “OK” hand gesture, adopted by those on the far-right to display a "sincere expression of white supremacy", according to the Anti-Defamation League.
At the first presidential debate in September, Mr Trump refused to denounce the Proud Boys. Instead, he told them to "stand back and stand by", comments later celebrated by the group’s leaders.
A number of violent clashes broke out between Trump supporters and counter-protesters in DC on Saturday following a march in support of the president’s false claims of voter fraud at the election.
Four people were stabbed in the capital, according to reports, but none were in a life-threatening condition. One person was also shot at a Trump protest in Olympia, Washington. The person’s condition was not immediately clear.
Mr Trump continues to falsely claim that he won the election, tweeting on Saturday that his team “HAVE JUST BEGUN TO FIGHT!!!!”
On Monday, the electoral college meets to certify the election results for Mr Biden as the 46th president.
Mr Trump has suffered a host of legal defeats to his campaign’s lawsuits aimed at overturning results in key battleground states by citing unsubstantiated claims of voter fraud.
While the transition process has already begun many Republican Party figures are rowing in behind the defeated incumbent.
More than 120 members of the House of Representatives formally asked the Supreme Court earlier this week to prevent four swing states from declaring Mr Biden the winner. The Supreme Court rejected the election challenge.
Some Republican officials have urged Mr Trump to dial down the rhetoric on his false voter fraud claims.
Gabriel Sterling, one of Georgia's top election officials, earlier this month called on the president to "stop inspiring people to commit potential acts of violence".
“Someone’s going to get hurt. Someone’s going to get shot. Someone’s going to get killed. And it’s not right," he added.
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