Four people died as supporters of President Donald Trump violently occupied the US Capitol.

Washington, DC, Police Chief Robert Contee said the dead on Wednesday included a woman who was shot by the US Capitol Police, as well as three others who died in "medical emergencies."

Ashli Babbit, who is from San Diego, died after rioters stormed the US Capitol to try and overturn the outgoigng president's electoral defeat.

"I really don't know why she decided to do this," Ms Babbit's unnamed mother-in-law told FOX 5 DC.

It remained unclear who shot the woman, but a witness said the shooting occurred after law enforcement authorities urged people to "get back."

Hundreds of Trump supporters entered the Capitol building on Wednesday in a bid to overturn his election defeat, forcing Congress to postpone a session that would have certified Democratic President-elect Joe Biden's victory in the November 3 election.

US Capitol put on lockdown as pro-Trump demonstrators storm the Capitol

A protester from New Jersey told Washington CBS affiliate WUSA he witnessed the woman being shot at the Capitol after police and other law enforcement officials warned people to get back.

"Well, we had stormed into the chambers inside, and there was a young lady who rushed to the windows; a number of police and Secret Service were saying, 'Get back, get down, get out of the way,'" he said. "She didn't heed the call and as we kind of raced up to grab people and pull them back, they shot her in the neck, and she fell back on me."

The violence, that forced lawmakers to flee the US Capitol and resulted in the death of one woman, disrupted the process of certification of Biden as the winner in the November 3 election. However, the Senate resumed the process more than six hours after the attack.

Trump's Twitter account was locked for 12 hours after he pushed baseless claims about the election, and the social media platform warned him of permanent ban if he didn't remove his rule-breaking tweets. "As a result of the unprecedented and ongoing violent situation in Washington, DC, we have required the removal of three @realDonaldTrump Tweets that were posted earlier today for repeated and severe violations of our Civic Integrity policy," Twitter said in a post. "If the Tweets are not removed, the account will remain locked," it added.
Facebook and the platform its owns - Instagram - also said Trump's pages will be blocked for 24 hours, according to Associated Press. Facebook's vice president of integrity Guy Rosen said, "This is an emergency situation and we are taking appropriate emergency measures, including removing President Trump's video."

 Biden called for restoration of "simple decency" and said, "The words of a president matter, no matter how good or bad that president is. At their best, the words of a president can inspire. At their worst, they can incite," he added. He called on Trump to "go on national television now, to fulfil his oath and defend the Constitution and demand an end to this siege."
    Former US President Barack Obama said Trump incited the violence. "History will rightly remember today's violence at the Capitol, incited by a sitting president who has continued to baselessly lie about the outcome of a lawful election, as a moment of great dishonor and shame for our nation," he said.