Speaking on Tuesday night, Trump called the election results a "fraud" and said he would be going to the US Supreme Court over them. Biden has called these calls "outrageous" and said his legal team was ready to prevent this.
Watch a live broadcast from Washington DC where activists are gathering in Black Lives Matter plaza for a march organised under the banner of "Shut Down the Attack on Democracy".
This comes as the vote count for the US presidential election which took place on 3 November continues, with both major rivals, Trump and Biden, making optimistic statements concerning their victory.
So far, Trump is winning Florida, Texas and Iowa, while Biden has taken the lead in Minnesota, New Hampshire, Arizona and Michigan.