Protesters surged up the Capitol steps shoving past barricades and officers in riot gear to penetrate the building. The mob - some of whom wore body armour - used chemical irritants to attack police, according to Washington Metropolitan Police Chief Robert Contee, reports BBC.
They shouted and waved pro-Trump, US and Confederate flags as they roamed the halls, demanding the results of the presidential election be overturned.
Members of Congress scrambled for cover under their seats as tear gas was fired. Several thousand National Guard troops, FBI agents and US Secret Service personnel were deployed to help overwhelmed Capitol police.
The apparent ease with which Trump supporters were able to act has raised serious questions about security. Two pipe bombs were recovered, one from the Democratic National Committee offices, not far from the Capitol, and one from the nearby Republican National Committee headquarters.
The occupation of the Capitol lasted more than three hours before the building was secured by law enforcement. At least 52 people have been arrested, the vast majority of them for violating the 18:00 to 06:00 curfew.
Washington is under an extended state of emergency until after Mr Biden's inauguration.
Responding to the violence, Mr Biden said the mob's activity "border[ed] on sedition" and that democracy was "under unprecedented assault".
"To storm the Capitol, to smash windows, to occupy offices on the floor of the United States Senate, rummaging through desks, on the House of Representatives, threatening the safety of duly elected officials. It's not protest; it's insurrection."
media captionJoe Biden: The scenes of chaos at the Capitol do not reflect a true America, do not represent who we are
Mr Trump then posted a recorded video on Twitter saying: "I know your pain. I know you're hurt... We had an election that was stolen from us. It was a landslide election, and everyone knows it, especially the other side. But you have to go home now. We have to have peace."
For the first time, Twitter froze Mr Trump's account, saying it would be locked for 12 hours. The social media giant demanded he delete three tweets that it said could stoke violence and threatening "permanent suspension". Facebook and Instagram followed suit.
Hours earlier, Mr Trump had addressed a "Save America Rally" outside the White House, where he encouraged supporters to keep contesting the election result: "Our country has had enough and we will not take it anymore."
media caption"We will never give up, we will never concede", Trump tells supporters
Political figures from around the world condemned the violence. In the US, former President Barack Obama said history would rightly remember the assault on the Capitol as "as a moment of great dishonour and shame for our nation".
Former President George W Bush said: "It is a sickening & heartbreaking sight. This is how election results are disputed in a banana republic - not our democratic republic."
There were also protests on Wednesday at state legislatures in Kansas, Georgia, Utah and on the other side of the country in Oregon and the north-western state of Washington.
