The call comes two months after Trump supporters smashed through windows and doors in an insurrection meant to halt the January 6 certification of Joe Biden's presidential victory.

There were no signs of disturbance on Thursday at the heavily secured building, with police and guardsmen on duty and the streets and grounds around the building closed off with razor-wire-topped fencing.

There was also no evidence of a large group heading to Washington despite the warning.

Still, the threat distressed law enforcement officials grappling with how best to secure the Capitol after rioters sent lawmakers fleeing the iconic building in January in a siege broadcast around the world.

Five people died, including a US Capitol Police officer and a woman shot by police.

Several investigations are underway into security and intelligence failures and Congress has asked for a long-term plan for when the National Guard eventually withdraws.

Right now, there are about 5200 remaining in DC, the last of the roughly 26,000 brought in for President Biden's inauguration, which went off without problem.

Members of both parties have complained that the fence encircling the Capitol seals off access to constituents and the general public, projecting an image at odds with the seat of American democracy.

The most recent threat appeared connected to a far-right conspiracy theory, mainly promoted by supporters of QAnon, that former president Donald Trump would rise again to power on March 4 and thousands would come to Washington to try to remove Democrats from office.

March 4 was the original presidential inauguration day until 1933, when it was moved to January 20.

Trump, however, was in Florida.

In Washington, on one of the warmest days in weeks, the National Mall was almost deserted, save for joggers, journalists and a handful of tourists trying to take photos of the Capitol dome through the fencing.

The House had been expected to have a light schedule but called off its session, staying in late on Wednesday to wrap up its work in part because of the threat.

The Senate remained in session on Thursday on Biden's big COVID-19 relief bill.

House Speaker Nancy Pelosi dismissed the "silliness" of the alleged plot to restore Trump.

Still, she said with "the threat of all the president's men out there, we have to ensure, with our security, that we are safe enough to do our job but not impeding" Congress.

Online chatter identified by authorities included discussions among members of the Three Percenters, an anti-government militia group, concerning possible plots against the Capitol on Thursday, according to law enforcement officials.

But federal agents found no significant increases in the number of hotel rooms being rented in Washington or in flights to the area, car rental reservations or buses being chartered.

Australian Associated Press