Police have warned of a plot in which 4,000 armed insurrectionists could descend on Washington in the coming days, encircling the US Capitol.
It was one of at least three armed plots detailed in a phone call between US Capitol Police and Democrat members of Congress, according to politicians on the call.
Meanwhile, it emerged that the FBI had warned extremists were planning for "war" the day before the storming of the Capitol on Jan 6.
FBI agents were aware that plotters had shared maps of the tunnels under the Capitol online, and arranged locations to rendez-vous in states including Kentucky and South Carolina before heading to Washington.
But still the National Guard was not called in beforehand.
The disclosure came as 15,000 National Guard troops were activated to protect the Capitol in the lead up to Joe Biden's inauguration on Jan 20.
Following requests from Democrats some National Guard members were being subjected to additional background checks to ensure they were not "sympathetic to domestic terrorists."
Chuck Schumer, the Democrat leader in the Senate, called for anyone in the mob that entered the Capitol on Jan 6 to be put on a no-fly list to stop them returning to Washington ahead of the inauguration.
He said: "We cannot allow these same insurrectionists to get on a plane and cause more violence and more damage."
Donald Trump declared a state of emergency in Washington and the city's Democrat mayor, Muriel Bowser, pleaded with Americans to stay away.
The FBI warned of armed protests at all 50 state Capitols in the coming days, and security was beefed up at government buildings across the country.
In a police phone call with members of Congress on Monday night politicians were taken through specific threats being made ahead of the inauguration.
One plot outlined by police was described as the "largest armed protest ever to take place on American soil."
Another was a protest planned to remember Ashli Babbitt, who was shot dead during the invasion of the Capitol last week.
A third would see thousands of armed people surrounding Congress to stop Democrat politicians from going in, which could result in assassinations.
Extremists might also surround the US Supreme Court and the White House, politicians were told.
Rules of engagement were being drawn up for how the National Guard should deal with armed insurrectionists.
Conor Lamb, a Democrat congressman from Pennsylvania, and former US Marine, who was on the phone call with police, said: "They were talking about 4,000 armed 'patriots' to surround the Capitol and prevent any Democrat from going in.
"They have published rules of engagement, meaning when you shoot and when you don't. So this is an organised group that has a plan."
He added: "They [insurrectionists] are committed to doing what they're doing because I think in their minds they are patriots and they're talking about 1776 and so this is now a contest of wills.
"We are not negotiating with or reasoning with these people. They have to be prosecuted. They have to be stopped. And unfortunately, that includes the president, which is why he needs to be impeached and removed from office. "The threats we are facing are very specific."
The internal FBI warning before Jan 6 was issued after agents picked up online chatter.
It included comments such as "Be ready to fight. Congress needs to hear glass breaking, doors being kicked in, and blood from their BLM and Pantifa [Antifa] slave soldiers being spilled.
"Get violent. Stop calling this a march, or rally, or a protest. Go there ready for war. We get our President or we die. NOTHING else will achieve this goal."
However, the memo, which was immediately passed to the FBI in Washington, also suggested that the words were protected by the First Amendment right to free speech.
The FBI said it had opened 160 case files and received 100,000 pieces of digital media as tips, and was "scouring every one."
The FBI did not rule out that insurrectionists planned to take members of Congress hostage.
Steven D'Antuono, assistant director of the FBI's Washington office, said: "We are looking at all angles, interviewing witnesses and subjects as they get arrested to ascertain the true purpose of some of the individuals in the Capitol that day."
He defended the handling of intelligence before the attack on Jan 6.
He said the FBI had to "separate the aspirational from the intentional" - "despicable things said on the internet" from "intent to do harm".
Michael Sherwin, the acting US Attorney, said the investigation was "unprecedented" and there had been charges in 70 cases. He expected hundreds of people to be charged eventually.
He said: "The Capitol is essentially a crime scene."
Mr Sherwin said he expected to prosecute cases of sedition with prison terms of up to 20 years.