The leaders of the two houses of the US Congress have called for President Donald Trump to be removed from office after the violent invasion of the Capitol by a mob of his supporters.

Democratic Senator Chuck Schumer said Mr Trump should be removed immediately. If he is not, House Speaker Nancy Pelosi says he could be impeached.

Four people died during the riot, and 68 have now been arrested.

Police have faced criticism for failing to stop the rioters breaking in.

The official responsible for the security in the House of Representatives, the sergeant at arms, has now resigned. Mr Schumer has called for his counterpart in the Senate to be sacked.

President-elect Joe Biden said: "Nobody could tell me that if it was a group of Black Lives Matter protesters yesterday they wouldn't have been treated very differently than the thugs that stormed the Capitol."

The head of the US Capitol Police (USCP) said his officers had faced "criminal riotous behaviour" and he described officers' actions as "heroic".

Ms Pelosi, however, has called on him to resign.

Transport Secretary Elaine Chao is the latest member of the Trump administration to quit in protest over the riots. Several lower-level officials have also resigned.

How have lawmakers responded?

A growing number have called for Mr Trump to be removed as president. Most of them are from Mr Biden's Democratic Party but one or two Republicans have joined in.

"This president should not hold office one day longer," said Mr Schumer, who will lead the Democratic majority in the Senate when it begins its new session later this month.

He threatened to start impeachment proceedings if Mr Trump's cabinet did not remove him from office, using the 25th Amendment to the US Constitution, which allows the vice-president to step up if the president is unable to perform his duties owing to a mental or physical illness.

It would require Vice-President Mike Pence and at least eight cabinet members to break with Mr Trump and invoke the amendment - something they have so far seemed unlikely to do.

Ms Pelosi described Mr Trump as "a very dangerous person" and said: "This is an emergency of the highest magnitude."

She left open the option of impeachment if Mr Trump's colleagues did not start 25th Amendment proceedings.

Mr Trump himself has remained silent about Wednesday's violence.

Congressman Adam Kinzinger from Illinois has become one of the first Republicans to call for use of the 25th Amendment, saying: "Fires stoked by the president finally leapt out of the pan."

The Republican governors of Maryland and Vermont have also called for Mr Trump to be removed from office.

Democrats on the House Judiciary Committee have said that Mr Trump's actions meet the amendment's requirements.

Separately, both the House and the Senate have adjourned until after Mr Biden is sworn in on 20 January.

Both houses would have to be recalled in order to start impeachment proceedings.

Elaine Chao is the most high-profile of several Trump administration officials resigning over the riots.

Ms Chao, who has served through the whole presidency and is married to the top Senate Republican, Mitch McConnell, said the events had "deeply troubled me in a way that I simply cannot set aside".

Special envoy Mick Mulvaney, senior national security official Matt Pottinger, and Stephanie Grisham, press secretary to First Lady Melania Trump, have also stepped down. Several others are reported to be on the verge of quitting.

The president is also facing new restrictions on his use of social media. Facebook, which owns Instagram, has suspended him from both platforms at least until he leaves office and possibly indefinitely.

Twitter suspended his account for 12 hours.

What was the role of the police?

Police have been heavily criticised for their response to the violence.

Images captured inside the Capitol building showed rioters strolling through some of the corridors unimpeded. One man occupied Ms Pelosi's office, putting his feet up on her desk, while another was photographed walking out carrying a lectern.

Washington police say 68 people have so far been arrested, only one of them from the DC area.

One USCP officer has been placed on leave after a woman was shot dead on the floor of the House of Representatives. The woman, who has been named as air force veteran Ashli Babbitt, 35, was part of a group that forced its way into the chamber while it was still in session.

The FBI is now seeking to identify those involved in the rampage.

On Thursday, crews began installing a non-scalable 7ft (2m) fence around the Capitol.

What have Biden and Trump said?

Mr Biden denounced what he called an "insurrection" and riotous activity which "borders on sedition".

Mr Trump, while he told the mob in a Twitter video to "go home", also continued to make false claims of electoral fraud and said he understood the "pain" of his supporters.

After the rioters left the Capitol, Congress reconvened to certify Mr Biden's victory. Mr Pence resisted pressure from Mr Trump to throw out some of the votes and change the election result.

Shortly afterwards Mr Trump said in a statement that, although he disagreed with the outcome of the election, "there will be an orderly transition on January 20th".

On Thursday the White House press office issued a statement saying it "grieves the loss of life" in Wednesday's violence and would continue "to pray for a speedy recovery for those who suffered injury".