Donald Trump faces trial in the Senate after becoming the first US president to be charged with misconduct in office for a second time.
Mr Trump is accused of inciting a mob that stormed Congress last week after he repeated false claims of election fraud. Five people died.
The trial will be held after the president leaves office next Wednesday.
If Mr Trump is convicted, senators could also vote to bar him from ever holding public office again.
The trial follows Wednesday's vote in the House of Representatives that formally charged - or impeached - the president with "incitement of insurrection" for his role in the riot.
The Republican president has rejected responsibility for the violence. In a video released by the White House after the vote, he called on his supporters to remain peaceful, without mentioning his impeachment.
The FBI has warned of possible armed protests planned for Washington DC and all 50 US state capitals in the days before Joe Biden, a Democrat, is inaugurated as the new US president.
What happens next?
The Senate - the upper house of the US Congress - will hold a trial to determine the president's guilt but this will not happen during Mr Trump's remaining week in office.
Republican Senate leader Mitch McConnell said there was "simply no chance that a fair or serious trial" could conclude given "the rules, procedures, and Senate precedents" that govern trials involving presidents.
A two-thirds majority will be needed to convict Mr Trump, meaning at least 17 Republicans would have to vote with Democrats in the evenly split, 100-seat chamber.
As many as 20 Republicans are open to convicting the president, the New York Times reported on Tuesday. In a note to colleagues, Mr McConnell said he had not made a final decision on how he would vote.
If Mr Trump is convicted, senators could then hold another vote to block him from running for elected office again, which he has indicated he planned to do in 2024.
Mr Trump was impeached by the House in 2019 over his dealings with Ukraine, but acquitted by the Senate.
Impeachment: The basics
- What is impeachment? Impeachment is when a sitting president is charged with crimes. In this case, President Trump was charged with inciting insurrection by encouraging his supporters to storm the Capitol
- Could Trump be removed from office? The House of Representatives has impeached him, moving the case to the Senate for a trial. But a trial will not be carried out before Mr Trump leaves office on 20 January
- So what does it mean? A trial can happen after his term ends, and senators can vote to bar him from holding public office again
What was Trump charged with?
Impeachment charges are political, not criminal. The president was accused by the House of inciting the storming of the Capitol - the seat of the US Congress - with a speech on 6 January to supporters outside the White House.
He urged them to "peacefully and patriotically" make their voices heard, but also to "fight like hell" against an election that he falsely told them had been stolen.
Following Mr Trump's remarks, his supporters broke into the Capitol, forcing lawmakers to suspend certification of election results and take shelter while the building was placed on lockdown.
The article of impeachment stated that Mr Trump "repeatedly issued false statements asserting that the presidential election results were fraudulent and should not be accepted".
It says he then repeated these claims and "wilfully made statements to the crowd that encouraged and foreseeably resulted in lawless action at the Capitol", leading to violence and loss of life.
"President Trump gravely endangered the security of the United States and its institutions of government, threatened the integrity of the democratic system, interfered with the peaceful transition of power, and imperilled a coequal branch of government."
Trump makes history once again
Donald Trump has made history once again, this time as the first president to be impeached twice.
A year ago, the move was opposed in lockstep by the Republican Party. This time, a handful of conservatives backed the move. It is a reflection not only of the gravity of the moment, but also the president's declining influence in the final days of his administration.
Impeachment sets up a Senate trial for Mr Trump that now appears destined to stretch into the early days of Joe Biden's presidency, creating yet another challenge for the incoming president. It also will stoke an ongoing debate among Republicans over the direction their party takes in the days ahead.
The party is on a path that splits in two very different directions. On one side is continued allegiance to the president's brand of politics - one that created a new coalition of voters that delivered the White House and Congress in 2016, but lost both in 2020.
On the other is an uncertain future - but one free from the president's unique style of heat and rhetoric - unfiltered invective that even many Republicans now believe contributed to last week's Capitol riot
What happened at the House vote?
For two hours, members of the Democratic-controlled House made statements for and against the vote on impeachment while National Guard troops kept watch inside and outside the Capitol.
House Speaker Nancy Pelosi, a Democrat, said: "The president of the United States incited this insurrection, this armed rebellion against our common country. He must go. He is a clear and present danger to the nation that we all love."
Most Republicans did not seek to defend Mr Trump's rhetoric, instead arguing that the impeachment had bypassed the customary hearings and calling on Democrats to drop it for the sake of national unity.
"Impeaching the president in such a short time frame would be a mistake," said Kevin McCarthy, the House's top Republican. "That doesn't mean the president's free from fault. The president bears responsibility for Wednesday's attack on Congress by mob rioters."
Mr Trump was impeached by 232-197, with ten Republicans siding with Democrats. Among them was the third-ranking House Republican, Liz Cheney, the daughter of former Vice-President Dick Cheney.
With a sombre and conciliatory tone, Mr Trump said in his video after the vote: "Violence and vandalism have no place in our country... No true supporter of mine would ever endorse political violence."
Mr Biden said he hoped senators would not neglect the "other urgent business of this nation", such as approving his cabinet nominees, coronavirus relief and the nationwide vaccination programme.