Calls mount for Donald Trump's immediate removal from office as Democrats draw up articles of impeachment

Senate minority leader Chuck Schumer said Mike Pence should invoke the 25th Amendment to the Constitution

The US Senate's top Democrat called on Thursday for President Donald Trump's immediate removal from office after his supporters stormed the Capitol in a shocking assault on the heart of American democracy.

"What happened at the US Capitol yesterday was an insurrection against the United States, incited by the president," Senate minority leader Chuck Schumer said in a statement. "This president should not hold office one day longer."

The senator from New York said Vice President Mike Pence should invoke the 25th Amendment to the Constitution, which allows for a majority of the cabinet to replace a president deemed unable to discharge his duties.

"If the Vice President and the Cabinet refuse to stand up, Congress should reconvene to impeach the president," he said.

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Representative Adam Kinzinger, a Republican from Illinois, also called for invoking the 25th Amendment, which has only been used before when a president has undergone a surgical procedure.

"It's time to evoke the 25th Amendment and end this nightmare," Kinzinger said. "The president is unfit. And the president is unwell."

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

A group of Democratic House lawmakers, led by Ilhan Omar and including Alexandria Ocasio-Cortez, separately announced they have drawn up a resolution on articles of impeachment ready for introduction.

The calls for Trump's removal came a day after his supporters stormed the Capitol in a violent attempt to prevent the certification by Congress of Democrat Joe Biden's election victory.

Trump responded to Congress's late night certification of Biden's election win by pledging an "orderly transition" -- but once again refused to concede, repeating the unfounded allegations of election fraud that fueled the mob assault on Congress.

Biden is to address reporters at 1:30 pm (1830 GMT) to announce the nomination as attorney general of Merrick Garland, a federal appeals court judge denied a seat on the US Supreme Court by Republicans five years ago.

Biden, 78, is expected to also address the violence in the nation's capital, which he has characterized as an "insurrection."

"Our democracy's under unprecedented assault," Biden said in his home state of Delaware, where he is preparing the transition to be sworn in as president on January 20.

In an angry, rambling speech outside the White House before the violence, Trump urged his supporters to march to the Capitol and demanded that Pence, who ceremonially led the session, intervene to reverse their defeat.

The vice president refused, and it was ultimately Pence standing before the joint session of Congress who announced his and Trump's loss to Biden and incoming Vice President Kamala Harris.

Mick Mulvaney, Trump's former chief of staff, announced following the violence he was resigning as the US special envoy to Northern Ireland and he expected others to step down.

"I can't stay here, not after yesterday," Mulvaney told CNBC television.

"And I wouldn't be surprised to see more of my friends resign over the course of the next 24 to 48 hours."

The certification of Biden's November 3 election win came at the end of a marathon 15-hour joint session of the House of Representatives and Senate interrupted for six hours by the mayhem at the Capitol and featuring Republican objections.

It formally put an end to any hopes the 74-year-old Trump may have had of reversing the result through weeks of false claims that he actually won.

Congress rejected challenges to Biden's win in Arizona and Pennsylvania and Republicans dropped objections to Biden's victories in other states after the storming of the Capitol.

Trump released a statement pledging an "orderly transition" but suggesting he would remain in frontline politics, amid speculation he may run again in 2024.

"Even though I totally disagree with the outcome of the election, and the facts bear me out, nevertheless there will be an orderly transition on January 20th," he said.

"While this represents the end of the greatest first term in presidential history, it's only the beginning of our fight to Make America Great Again!"

Egged on by an aggrieved Trump, a flag-waving mob hours earlier broke though barricades and swarmed inside the Capitol, ransacking offices and entering the usually solemn legislative floors.

Security forces fired tear gas in a four-hour operation to clear the Capitol. Police said a woman, reportedly a Trump partisan from southern California, was shot and killed and three others died in the area in circumstances that were unclear.

One Trump backer in jeans and a baseball cap was pictured propping a leg up on House Speaker Nancy Pelosi's desk, as throngs climbed onto risers set up for Biden's January 20 inauguration.

Former president Barack Obama called it "a moment of great dishonor and shame for our nation" and said it was "incited" by Trump, "who has continued to baselessly lie about the outcome of a lawful election."

Several hours after the violence began, Trump released a video urging the crowd to leave but repeating his unfounded claims of election fraud.

Social media firms removed Trump's video on charges it aggravated violence and Twitter temporarily suspended his account, while Facebook went further Thursday announced it was banning Trump "indefinitely."

The chaos at the Capitol came a day after Biden enjoyed a new triumph with Democrats projected to win two Senate seats in runoffs in Georgia -- handing the party full control of Congress and dramatically increasing Biden's ability to pass legislation.

Historians said it was the first time that the Capitol had been taken over since 1814 when the British burned it during the War of 1812.