The riot by Trump supporters who breached the security of Congress on Wednesday was "not dissent, was not disorder, was not protest. It was chaos".

Those who massed on Capitol Hill intending to disrupt a joint session of Congress that was certifying Biden's election victory over Trump "weren't protesters. Don't dare call them protesters. They were a riotous mob - insurrectionists, domestic terrorists. It's that basic", Biden said on Thursday.

In solemn tones, Biden said the actions Trump has taken to subvert the nation's democratic institutions throughout his presidency led directly to the mayhem in Washington.

"In the past four years, we've had a president who's made his contempt for our democracy, our Constitution, the rule of law clear in everything he has done," Biden said.

"He unleashed an all-out assault on our institutions of our democracy from the outset. And yesterday was the culmination of that unrelenting attack."

The mob of hundreds of Trump backers broke into the Capitol and roamed the halls looking for lawmakers, who were forced to halt their deliberations and seek safety. The violent protesters were egged on by Trump himself, who has falsely contended that he lost the election due to voter fraud.

Trump's claims were repeatedly dismissed in the courts, including the Supreme Court, and by state election officials from both parties, and even by some in his own administration.

But the president went to greater and greater lengths to try to subvert the election, culminating this week in efforts by some Republican members of Congress to object to the certification of the results and the violence at the Capitol.

After the disruption, Congress returned to work late on Wednesday and affirmed Biden's victory early on Thursday.

Biden ticked off a list of Trump's assaults on American norms, including his attacks on the press and the intelligence community and his pressure on state and federal officials and judges to submit to his efforts to overturn the election.

Biden said that on Wednesday, Trump tried to "use a mob to silence the voices of nearly 160 Americans" who voted in November.

And both he and Vice President-elect Kamala Harris spoke about the police treatment of the largely white protesters on Wednesday, compared with the heavy-handed way in which police have handled Black Lives Matter protests.

"We witnessed two systems of justice, when we saw one that let extremists storm the United States Capitol and another that released tear gas on peaceful protesters last summer," Harris said.

Biden declared that "no one can tell me that if it had been a group of Black Lives Matter protesting yesterday, they wouldn't have been treated very, very differently than the mob of thugs that stormed the Capitol."

He expressed hope that images comparing the police presence Wednesday to that marshalled to prepare for Black Lives Matters protests would open Americans' eyes to the needs for reform.

The remarks came during an event in Wilmington, Delaware, to introduce Biden's Justice Department team, to be led by federal appeals court judge Merrick Garland as attorney general.

Biden also announced Obama administration homeland security adviser Lisa Monaco would serve as deputy attorney general and former Justice Department civil rights chief Vanita Gupta as associate attorney general, the No.3 official.

He also named an assistant attorney general for civil rights, Kristen Clarke, now the president of Lawyers' Committee for Civil Rights Under Law, an advocacy group.

Australian Associated Press