US President Joe Biden has slammed his predecessor Donald Trump for watching his supporters riot for hours on 6 January, while police at the US Capitol suffered through a "medieval hell".

"Brave law enforcement officers were subject to the medieval hell for three hours, dripping in blood, surrounded by carnage, face to face with the crazed mob that believed the lies of the defeated president," Mr Biden told the National Organisation of Black Law Enforcement Executives Conference.

"For three hours, the defeated former president of the United States watched it all happen as he sat in the comfort of the private dining room next to the Oval Office," he said.

"The police were heroes that day. Donald Trump lacked the courage to act," Mr Biden said, adding: "You can't be pro-insurrection and pro-cop. You can't be pro-insurrection and pro-democracy. You can't be pro-insurrection and pro-American."

His criticism of Mr Trump echoed the conclusion of the House committee investigating the 6 January, 2021 riot at the Capitol, which on Thursday laid out a searing, prime-time indictment of the former president's refusal to halt or condemn the violence.

The pro-police remarks by Mr Biden - who is under fire from Republicans over a surge in violence in the United States - come as he seeks to burnish his credentials on crime ahead of November midterm elections, which are forecast to be painful for the Democratic Party.