US President-elect Joe Biden said Thursday that Donald Trump had incited one of the "darkest days" in US history, a day after pro-Trump rioters smashed their way into the Capitol.

Biden assailed the record of the outgoing president, saying the Republican had "unleashed an all-out assault on the institutions of our democracy from the outset" of his four years in office.

But the president-elect heaped specific scorn on Trump for encouraging his supporters not to accept his defeat at the polls, which ended in deadly chaos at the Capitol.

Hundreds of protesters broke down barriers and doors to invade the seat of the US legislature, an unprecedented act that played out on live television.

"Yesterday, in my view, was one of the darkest days in the history of our nation -- an unprecedented assault on our democracy," Biden said.

He accused authorities of treating pro-Trump protesters more leniently than anti-racism demonstrators who were forcibly dispersed by police in Washington and other cities last year.

The summer protests in Washington in particular were met with a large force of federal law enforcement and National Guardsmen.

"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," Biden said.

"We all know that's true, and it is unacceptable," he said.

The attack on the Capitol, where Biden formerly served for decades as a senator, took place two weeks before he is to take office, following his November 3 election victory over Trump.

Trump and his supporters maintain, without evidence, that Biden's win was rooted in massive fraud.