Police also warned of a conspiracy in which, in the coming days, 4,000 armed demonstrators might converge on Washington, surround the U.S. Capitol, and possibly assassinate politicians.

According to lawmakers who were on the call, it was one of three terrorist plots picked up by security services that were outlined in a conference call between the US Capitol Police and Democratic members of Congress.

It also emerged that the FBI had warned the day before the storming of the Capitol on January 6 that extremists were preparing a "war."

FBI agents were aware that plotters had online exchanged maps of the tunnels under the Capitol, and planned rendezvous locations before heading to Washington in states like Kentucky and South Carolina.

The National Guard, however, was still not called up beforehand.

In the run up to Joe Biden's inauguration next Wednesday, the revelations came as 15,000 National Guard troops were activated to defend the Capitol.

Any National Guard members were subjected to additional background checks following calls from Democrats to ensure they were not "sympathetic to domestic terrorists."

Meanwhile, Senate Democratic leader Chuck Schumer called for those in the crowd who entered the Capitol on January 6 to be placed on a no-fly list to avoid them coming back to Washington before the inauguration.

"We can't let these same insurrectionists get on a plane and cause more violence and more harm," he said.

In Washington, Donald Trump declared a state of emergency, and the Democratic mayor of the capital, Muriel Bowser, pleaded with Americans to stay away.

On Monday night, in a police conference call with members of Congress, lawmakers were warned of a series of threats made prior to the inauguration and identified by the police. One plot outlined was described as the "largest armed protest on American soil ever to take place."

Another was a rally scheduled to remember Ashli Babbitt, who died last week after being shot during the Capitol invasion. A third will see thousands of armed people surrounding Congress to prevent the entry of Democratic politicians, which could lead to assassinations.

The US Supreme Court and the White House may even be surrounded by extremists, politicians were told. Laws of engagement on how the National Guard can deal with armed insurrectionists have been drawn up.

Conor Lamb, a Pennsylvania Democratic congressman and former U.S. Marine, who was on the police phone call, said, "They were talking about 4,000 armed 'patriots' surrounding the Capitol, preventing any Democrat from coming in." They have engagement rules written, meaning when you shoot, and when you don't. They are committed to doing what they do because, I believe, they are patriots in their minds and they're talking about 1776. But now it's a battle of wills.

The internal FBI alert before January 6 was given after online contact was picked up by agents. It included remarks such as, "Be prepared to fight." Congress wants to hear glass shattering, doors being kicked in, and blood being poured by their BLM and [Antifa] slave troops.