Security officers in dark suits drew their pistols and trained them on the shouting mob trying to smash their way through the door of the normally sedate House of Representatives chamber.
The extraordinary scene unfolded amid the unprecedented assault on the US Congress supporters of President Donald Trump on Wednesday local time.
Protesters were angered by Trump's loss to his Democratic opponent Joe Biden in the November presidential election.
One woman was shot and killed in a chaotic crush of people near a stairway. A video posted on social media showed what sounded like a gunshot and an officer in dark clothing armed with a rifle standing near where she fell. Other officers rushed to assist her. The circumstances of her shooting, including who fired the shots, were not clear.
Three other people also died because of medical emergencies, and 14 police officers were injured - two of whom had to be hospitalised.
More than 52 people have been arrested - 47 of them for curfew violations.
Several others were arrested on charges related to carrying unlicensed or prohibited firearms.
Also, two pipe bombs were recovered from the headquarters of the Republican and Democratic national committees, as well as a cooler from a vehicle on US Capitol grounds that contained Molotov cocktails.
What began as a raucous protest against Trump's defeat boiled over into a siege of the Capitol, a symbol of US democracy, that paralysed Congress as it went through the final steps of ratifying the victory of President-elect Joe Biden.
Rioters brawled with police officers, pushed their way past metal security barricades, broke windows and forced their way inside the building, forcing lawmakers in both the House and the Senate to suspend their work and instead go into hiding in offices and other secured rooms.
A grey-bearded protester used what looked like a plexiglass riot shield to break a glass window of the Capitol, clearing the way for rioters to climb through.
Members with military experience, including Democratic Representative Jason Crow, a former US Army Ranger, and Republican Representative Markwayne Mullin, a former mixed martial arts fighter, jumped in to help.
Representative Reuben Gallego, a former Marine, told fellow House members how to put on their gas masks.
Much of the melee was captured in photos and videos posted on social media, news footage and witnessed by Reuters journalists.
The rioters who stormed the Capitol roamed freely through the corridors, carrying flags of at least one a large Confederate battle ensign and snapping photographs.
One man posed for a photo in the office of House Speaker Nancy Pelosi, the top Democrat in Congress, one booted foot propped on the corner of her desk. Another grinned at a news photographer as he carted a lectern down a hallway.
As demonstrators pressed closer to lawmakers, they scuffled with officers. One group discharged a fire extinguisher, leaving a haze. Tear gas blew into the rotunda beneath the iconic building's soaring dome.
Some rioters besieged the House of Representatives chamber while lawmakers were inside, banging on its doors, cutting off escape routes.
"It ... felt like it was happening in slow motion," Democratic Representative Eric Swalwell said.
Security officers piled furniture against the chamber's door and drew their pistols.
Lawmakers and reporters heard loud popping noises from outside and dived for cover on the floor.
"I was opening my gas mask and we heard 'pop, pop, pop' and we all went down," Democratic Representative Vicente Gonzalez told Reuters.
After 15 minutes, Capitol Police officers secured enough of a pathway through the Capitol to escort House members from the chamber to another location, urging them to hurry as they wound down stairs and through tunnels to avoid the chaos.
Many House members were in their offices when the assault happened because Pelosi had limited the number of people in the chamber to 11 from each party because of concerns about the coronavirus pandemic.
Some evacuated at the urging of Capitol police while others barricaded themselves inside their offices
Representative Mike Gallagher, a Republican from Wisconsin, said in a video on social media that he was "sheltering in place" in his office.
"This is banana republic crap that we're watching happen right now," he said.
Across the Capitol, other demonstrators pushed into the Senate gallery, where minutes earlier security officials had evacuated lawmakers and Vice President Mike Pence, who presided over the vote-counting.
A few climbed down from the gallery to the chamber floor, then climbed to the dais for photographs.
One man, bare-chested and wearing a fur hat with horns, his face painted red, white and blue, posed for photos there holding an American flag.
Dozens of lawmakers hunkered in a secure room for hours while officers worked to clear the building. Lawmakers from both parties, riven hours earlier by the election outcome, prayed together for American democracy.
When Congress returned on Wednesday night to resume counting the votes, clusters of armed agents, some wearing tactical gear and carrying automatic weapons, were stationed around the Capitol.