Describing the attack on Capitol Hill by Donald Trump's supporters as one of the darkest days in the US history, President-elect Joe Biden said the violence was the result of the President's "contempt for democracy".

The riot at the US Capitol on Wednesday was not dissent, disorder or protest, it was chaos, Biden told reporters on Thursday in Wilmington, Delaware.

"They weren't protesters, don't dare call them protesters. They were a riotous mob, insurrectionists, domestic terrorists...I wish we could say we couldn't see it coming but that isn't true. We could see it coming. The past four years we have had a president who has made his contempt for our democracy, our Constitution, the rule of law, clear in everything he has done, he said.

Biden alleged Trump has unleashed an all-out assault on institutions of US democracy from the outset and the Capitol Hill violence was "the culmination of that unrelenting attack".

The incident was "one of the darkest days in the history of our nation", Biden said, underlining that it was an unprecedented assault on the US democracy, on the citadel of liberty and on the rule of law.

Trump, he said, has always attacked the free press who dared to question his power, repeatedly calling them the enemy of the people, a language long been used by autocrats and dictators all over the world to hold on to power.

Biden said Trump thought he could stack the courts with friendly judges who would support him no matter what but "he was stunned when the judges he appointed didn't do his bidding, instead acted with integrity...upholding the rule of law not just once or twice or three times but over 60 times".

At every level, the judiciary rose in the moment during this election, did its job, acted with complete fairness and impartiality with complete honour and integrity, he said.

Trump was trying to use a mob to silence the voices of nearly 160 million Americans who summoned the courage in the face of a pandemic that threatened their health and their lives to cast that sacred ballot, he said.