Rioters storm Nancy Pelosi's congressional office, and others

Several photos showed protesters posing at a desk in Pelosi's office, holding papers, and leaving behind notes

A supporter of US President Donald Trump at a desk in the office of US House Speaker Nancy Pelosi, after supporters of the president breached the US Capitol security in Washington, DC. EPA

The office of Democratic House Speaker Nancy Pelosi was briefly taken over by some of the hundreds of pro-Trump protesters who surged past security and entered the Capitol building in Washington on Wednesday.

Photos showed protesters posing at a desk in Ms Pelosi's office, holding papers, and leaving behind notes.

"We will not back down," read one of the notes scrawled in all caps on a manila folder.

One man, dressed in a plaid shirt, even went as far as to put his feet on a desk in the House Speaker's office, grinning and stretching his arms out while posing for photographs.

According to news station KNWA, the man in Ms Pelosi's office is Richard Barnett of Gravette, Arkansas.

Mr Barnett later told KNWA that he was tear-gassed and escorted out of the Capitol building, and will soon hire a lawyer to deal with threats from his photo being widely circulated.

Although some of the protesters appeared to be smiling while posing for pictures, the Federal Bureau of Investigation did not see any humour in the security breach.

"The FBI is accepting tips and digital media depicting rioting and violence in the US Capitol building and surrounding area in Washington," the agency said.

The FBI tweeted a similar message to at least 110,000 followers, encouraging people to submit photos or videos that might help investigators lead to arrests.

Ms Pelosi was not the only official to have her office ransacked.

A video circulating on social media showed the US Senate's parliamentarian office in disarray following the surge of unruly demonstrators.

Demonstrators could also be seen breaking windows, knocking down doors and carrying various flags, some of them Confederate flags throughout the building.

Shattered glass could be seen on the marble floors in some photos.

At least four people died as a result of the demonstrations.

Washington DC police confirmed that at least one woman was fatally shot inside the Capitol by an officer, and three others suffered medical incidents just outside, dying later as a result.

The demonstrations sought to put a stop to the certification of Joe Biden's victory by Congress.

Some congressional leaders and former presidents blamed President Donald Trump for ratcheting up the crowd and inciting violence by continuing to push unproven and unfounded vote fraud theories when he spoke at a rally near the White House on Wednesday.

At a news conference, Washington DC police chief Robert Contee said that at least 50 people had been arrested as a result of the protests, and that 14 police officers were injured.

"These officers should be commended for their work, they fought hard to defend our democracy," Mr Contee said. "It was a chaotic scene, one that we had not experienced in this city before."

Despite the violent efforts to disrupt the process, Vice President Mike Pence later reconvened Congress and presided over the final electoral vote certification .

Mr Biden and vice president-elect Kamala Harris will be sworn in on January 20.