The centre of Washington was in lockdown Thursday as more than 20,000 armed National Guard troops were mobilised following the deadly assault on the Capitol and the threat of more violence at the presidential inauguration of Joe Biden.
Police chief Robert Contee said the US capital was facing "a major security threat" after supporters of outgoing President Donald Trump stormed the Capitol building a week ago in a violent bid to block Biden's confirmation that left five people dead.
A day after Trump was impeached in Congress for allegedly supporting the assault, more barriers were being erected and razor wire laid as part of precautions ahead of the January 20 ceremony.
Most of downtown Washington was off-limits to traffic, drawing comparisons to Baghdad's high-security "Green Zone."
The Secret Service, in charge of security, was weighing an unprecedented shutdown of the entire National Mall, the grassy spread where hundreds of thousands traditionally gather to celebrate the inauguration of a new president.
Security officials are warning that armed extremist Trump supporters, possibly carrying explosives, pose a threat to Washington as well as state capitals over the coming week.
"We are seeing an extensive amount of concerning online chatter," FBI Director Chris Wray told Vice President Mike Pence in a televised briefing.
"We're concerned about the potential for violence at multiple protest rallies planned here in DC and state capitol buildings around the country in the days to come, that could bring armed individuals within close proximity to government buildings and officials," he said.
In steps not seen since after the September 11, 2001 national alerts, an internal FBI bulletin warned that an armed group planned to "storm" government offices in all 50 states to protest Biden, ABC News reported.
"The FBI received information about an identified armed group intending to travel to Washington, DC on 16 January," the bulletin added.
The New York Times reported that the FBI has told police departments around the country to stay alert for extremist activity and pass on intelligence.
Political figures boosted their own personal security, as Washington Mayor Muriel Bowser revealed she has received death threats.
Peter Meijer, one of 10 House Republicans who voted with Democrats on Wednesday to impeach Trump for supporting "insurrection," said he and other lawmakers were taking precautions like acquiring body armour.
"I have colleagues who are now traveling with armed escorts out of the fear for their safety," he told MSNBC. "Our expectation is that someone may try to kill us."
The potential threat was underscored earlier this week with the discovery that one of those arrested for taking part in the January 6 attack on the Capitol had the kind of weapons and ammunition one might use for an assassination.
Court documents said Cleveland Grover Meredith had two guns, one with a telescopic sight, and around 1,000 rounds.
Another man was arrested with four guns and 11 Molotov cocktails in his truck.
Police are still searching for a man who planted two pipe bombs near the Capitol on January 6.
On Tuesday police in Chicago arrested a man who weeks ago phoned in threats against Biden.
Worries were heightened when The Washington Post reported that dozens of people on an FBI terrorist watch list were in Washington for the pro-Trump rallies on January 6.
For the upcoming week Washington officials have urged people to stay home rather than celebrate the inauguration.
Downtown hotels were closing their doors, and Airbnb has cancelled reservations in the days around the event.
Airlines were tightening security, with Delta announcing that legally armed travellers formerly allowed to check their guns at security would not even be allowed to do that on flights to Washington.
In several states, National Guard were also called out to prepare security for January 16-20.
Washington State, Michigan and others said they were concerned about possible far-right violence.