WASHINGTON (AP) — A violent mob loyal to President Donald Trump stormed the U.S. Capitol on Wednesday and forced lawmakers into hiding, in a stunning attempt to overturn America’s presidential election, undercut the nation’s democracy and keep Democrat Joe Biden from replacing Trump in the White House.

The nation’s elected representatives scrambled to crouch under desks and don gas masks, while police futilely tried to barricade the building, one of the most jarring scenes ever to unfold in a seat of American political power. A woman was shot and killed inside the Capitol, and Washington’s mayor instituted an evening curfew in an attempt to contain the violence.

The rioters were egged on by Trump, who has spent weeks falsely attacking the integrity of the election and had urged his supporters to descend on Washington to protest Congress’ formal approval of Biden’s victory. Some Republican lawmakers were in the midst of raising objections to the results on his behalf when the proceedings were abruptly halted by the mob.

Together, the protests and the GOP election objections amounted to an almost unthinkable challenge to American democracy and exposed the depths of the divisions that have coursed through the country during Trump’s four years in office. Though the efforts to block Biden from being sworn in on Jan. 20 were sure to fail, the support Trump has received for his efforts to overturn the election results have badly strained the nation’s democratic guardrails.

Congress reconvened in the evening, lawmakers decrying the protests that defaced the Capitol and vowing to finish confirming the Electoral College vote for Biden’s election, even if it took all night. Lawmakers were still at it after midnight.

Vice President Mike Pence, reopening the Senate, directly addressed the demonstrators: “You did not win.”

Republican Senate Majority Leader Mitch McConnell said the “failed insurrection” underscored lawmakers’ duty to confirm the vote. Democratic House Speaker Nancy Pelosi said Congress would show the world “what America is made of” by finishing the count.

Punctuating their resolve, both the House and Senate soundly defeated the first objection — to election results from Arizona — that had been raised by Sen. Ted Cruz, R-Texas, and Rep. Paul Gosar, R-Ariz. Still, most House Republicans voted for the objection.

Other objections to results from Georgia, Michigan and Nevada fizzled without adequate support from senators. But an objection to Pennsylvania backed by Sen. Josh Hawley, R-Mo., and Rep. Scott Perry, R-Pa., forced deliberations. Proceedings pushed into the wee hours.

The president gave his supporters a boost into action Wednesday morning at a rally outside the White House, where he urged them to march to the Capitol. He spent much of the afternoon in his private dining room off the Oval Office watching scenes of the violence on television. At the urging of his staff, he reluctantly issued a pair of tweets and a taped video telling his supporters it was time to “go home in peace” — yet he still said he backed their cause.

Hours later, Twitter for the first time time locked Trump’s account, demanded that he remove tweets excusing violence and threatened “permanent suspension.”

A somber President-elect Biden, two weeks away from being inaugurated, said American democracy was “under unprecedented assault, ” a sentiment echoed by many in Congress, including some Republicans. Former President George W. Bush said he watched the events in “disbelief and dismay.”

The domed Capitol building has for centuries been the scene of protests and occasional violence. But Wednesday’s events were particularly astounding both because they unfolded at least initially with the implicit blessing of the president and because of the underlying goal of overturning the results of a free and fair presidential election.

Tensions were already running high when lawmakers gathered early Wednesday afternoon for the constitutionally mandated counting of the Electoral College results, in which Biden defeated Trump, 306-232. Despite pleas from Republican Senate Majority Leader Mitch McConnell, more than 150 GOP lawmakers planned to support objections to some of the results, though lacking evidence of fraud or wrongdoing in the election.

Trump spent the lead-up to the proceedings publicly hectoring Pence, who had a largely ceremonial role, to aid the effort to throw out the results. He tweeted: “Do it Mike, this is a time for extreme courage!”

But Pence, in a statement shortly before presiding, defied Trump, saying he could not claim “unilateral authority” to reject the electoral votes that make Biden president.

In the aftermath, several Republicans announced they would drop their objections to the election, including Sen. Kelly Loeffler, R-Ga., who lost her bid for reelection Tuesday.

Earlier, protesters had fought past police and breached the building, shouting and waving Trump and American flags as they marched through the halls. Lawmakers were told to duck under their seats for cover and put on gas masks after tear gas was used in the Capitol Rotunda. Some House lawmakers tweeted they were sheltering in place in their offices.

Rep. Scott Peters, D-Calif., told reporters he was in the House chamber when rioters began storming it. Security officers “made us all get down, you could see that they were fending off some sort of assault." He said they had a piece of furniture up against the door. "And they had guns pulled,” Peters said. Glass panes to a House door were shattered.

The woman who was killed was part of a crowd that was breaking down the doors to a barricaded room where armed officers stood on the other side, police said. She was shot in the chest by Capitol Police and taken to a hospital where she was pronounced dead. City police said three other people died from medical emergencies during the long protest on and around the Capitol grounds.

Staff members grabbed boxes of Electoral College votes as the evacuation took place. Otherwise, said Sen. Jeff Merkley, D-Ore., the ballots likely would have been destroyed by the protesters.

The mob’s storming of Congress prompted outrage, mostly from Democrats but from Republicans as well, as lawmakers accused Trump of fomenting the violence with his relentless falsehoods about election fraud.

“Count me out,” said Trump ally Sen. Lindsey Graham, R-S.C. “Enough is enough.”

Several suggested that Trump be prosecuted for a crime or even removed under the Constitution's 25th Amendment, which seemed unlikely two weeks from when his term expires.

“I think Donald Trump probably should be brought up on treason for something like this,” Rep. Jimmy Gomez, D-Calif., told reporters. “This is how a coup is started. And this is how democracy dies.”

Sen. Ben Sasse, R-Neb., who’s at times clashed with Trump, issued a statement saying, “Lies have consequences. This violence was the inevitable and ugly outcome of the President’s addiction to constantly stoking division.”

Despite Trump’s repeated claims of voter fraud, election officials and his own former attorney general have said there were no problems on a scale that would change the outcome. All the states have certified their results as fair and accurate, by Republican and Democratic officials alike.

The Pentagon said about 1,100 District of Columbia National Guard members were being mobilized to help support law enforcement at the Capitol. More than a dozen people were arrested.

As darkness fell, law enforcement officers worked their way toward the protesters, using percussion grenades to clear the area around the Capitol. Big clouds of tear gas were visible. Police in full riot gear moved down the steps, clashing with demonstrators.

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Associated Press writers Jill Colvin, Kevin Freking, Alan Fram, Matthew Daly, Ben Fox and Ashraf Khalil in Washington and Bill Barrow in Atlanta contributed to this report.