Donald Trump for the first time Thursday promised a smooth transition to Joe Biden and acknowledged his presidency was ending as calls grew for his removal from office for encouraging a mob attack on the US Capitol.

Trump supporters
Supporters of US President Donald Trump gather for a rally in Washington Photo: AFP / Brendan Smialowski

An unusually tame Trump, in a video he released on Twitter after a temporary suspension, condemned rioters who rampaged in his name through a congressional session that certified Biden's victory, although he did not go so far as to congratulate or even say the name of his successor.

"This moment calls for healing and reconciliation," said Trump, in a jarring shift of tone a day after a grievance-fueled outdoor rally in which he encouraged thousands of supporters to march on the Capitol.

"We have just been through an intense election and emotions are high, but now tempers must be cooled and calm restored," said Trump, standing before a lectern with the presidential seal.

"A new administration will be inaugurated on January 20. My focus now turns to ensuring a smooth, orderly and seamless transition of power," he said.

"Serving as your president has been the honor of my lifetime," Trump said, without explicitly conceding and insisting he was "fighting to defend American democracy."

Trump Supporters
Hundreds of supporters of US President Donald Trump gather in Washington, DC, a day before a protest called by the outgoing US president who refuses to concede defeat in November 2020's election. The US Congress will meet Wednesday to certify Democrat Joe Biden's electoral victory over Trump. Photo: AFPTV / Gilles CLARENNE

Trump's turnaround came as aides including one cabinet member resigned and the two top Democrats in Congress urged his immediate removal, fearing the damage he can still inflict in his less than two weeks left in the world's most powerful job.

Biden, who won seven million votes more than Trump in the November 3 election as well as a decisive edge in the vital state-by-state Electoral College, declined to address demands for Trump's removal but accused him of an "all-out assault on the institutions of our democracy."

"Yesterday, in my view, was one of the darkest days in the history of our nation," Biden said at an event to introduce his nominee for attorney general, respected judge Merrick Garland, who if confirmed will quickly need to decide whether to prosecute Trump.

Trump Supporters
Crowds of people gather as US President Donald Trump speaks to supporters before Congress meets to certify Joe Biden's election win Photo: AFP / MANDEL NGAN

"They weren't protesters," Biden said. "They were a riotous mob, insurrectionists, domestic terrorists."

"I wish we could say we couldn't see it coming but that isn't true," Biden said. "We could see it coming."

"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," he said.

House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer urged Vice President Mike Pence to invoke the 25th Amendment, which allows a majority of the cabinet to remove a president deemed unable to discharge his duties.

They threatened otherwise to impeach Trump for an unprecedented second time in hopes that the Senate, where Democrats are projected to have won control after runoff elections Tuesday in Georgia, will now oust him.

"This is an emergency of the highest magnitude," Pelosi said, describing Trump as a "very dangerous person."

"By inciting sedition, as he did yesterday, he must be removed from office," she said. "While it's only 13 days left, any day can be a horror show for America.

Few Republicans came forward to back such remedies although Representative Adam Kinzinger, a frequent Trump critic within his party, said it was time to "end this nightmare" and also called for invoking the 25th Amendment, which has been used previously when presidents undergo a surgical procedure.

"The president is unfit," Kinzinger said. "And the president is unwell."

Invoking the amendment would make Pence the acting president for the remaining two weeks the administration has in office.

Speaking to CNN, retired Marine Corps general John Kelly, who served as Trump's chief of staff for 18 months, said the cabinet should consider the 25th Amendment but believed the president had been chastened.

"He can give all the orders he wants but no one is going to break the law," Kelly said.

Pence, loyally by Trump's side until the final days, rejected Trump's vocal pressure to somehow intervene in Tuesday's session, which has taken place every four years for more than two centuries without drama.

In the middle of the night, after hours of delay due to the riots and Trump loyalists' contesting of the results, it was Pence who formally announced the victory of Biden as the 46th president and Kamala Harris as the next vice president.

Transportation Secretary Elaine Chao, who is married to Republican Senate leader Mitch McConnell and is one of Trump's longest-serving cabinet members, announced she would resign over the "entirely avoidable" violence at the Capitol.

Thursday evening Education Secretary Betsy DeVos became the second cabinet member to quit, telling Trump in a letter that such "behavior was unconscionable for our country."

Others who resigned included Mick Mulvaney, a former Trump chief of staff who is now US special envoy to Northern Ireland, and the deputy national advisor, Matt Pottinger, an architect of Trump's hawkish line on China.

During the mayhem in Congress, security forces fired tear gas to drive out the flag-waving crowds and police said a woman, reportedly a Trump supporter from California, was shot by police and killed. Three other deaths were reported on the Capitol grounds but the circumstances remained unclear.

Bipartisan anger was brewing over the failure of law enforcement to prevent the mobs from entering Congress.

Steven Sund, the chief of the 2,300-strong Capitol Police, handed in his resignation and lawmakers vowed a thorough investigation on security lapses.

Many questioned how police would have responded had the crowd been not overwhelmingly white Trump supporters but Black anti-racism protesters, who were met with force in nationwide demonstrations last year.

"True progress will be possible only once we acknowledge that this disconnect exists and take steps to repair it," said former first lady Michelle Obama.

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