Supporters of sitting US President Donald Trump gather at Freedom Plaza in Washington, D.C., the United States, Jan. 5, 2020. (Xinhua/Liu Jie)

The US Capitol was breached by protesters on Wednesday afternoon, a tense situation that has forced proceedings to count Electoral College votes cast in the 2020 presidential election to halt.

Lawmakers are reportedly being evacuated from the House and the Senate, as protesters supporting outgoing President Donald Trump, a Republican, forced their way into the building, which has been placed under a lockdown, and entered the chamber.

"All buildings within the Capitol Complex, Capitol: External security threat, no entry or exit is permitted, stay away from exterior windows, doors. If outside, seek cover," according to a message from Capitol Police.

The US Congress convened in a joint session earlier in the day to certify President-elect and Democrat Joe Biden's victory over Trump.

Washington, D.C. Mayor Muriel Bowser on Wednesday afternoon ordered a citywide curfew in the wake of the incidents.

In a statement, Bowser said the curfew will run from 6 pm on Wednesday until 6 am on Thursday.

"During the hours of the curfew, no person, other than persons designated by the Mayor, shall walk, bike, run, loiter, stand, or motor by car or other mode of transport upon any street, alley, park, or other public place within the District," Bowser said.