Authorities have geared up for pro-Trump marches in all 50 state capitals, erecting barriers and deploying thousands of National Guard troops to try to prevent the kind of violent attack that rattled the nation on January 6.

The FBI warned police agencies of possible armed protests outside all 50 state capitol buildings starting Saturday through to Biden's inauguration on Wednesday, fuelled by supporters of President Donald Trump who believe his false claims of electoral fraud.

Michigan, Virginia, Wisconsin, Pennsylvania and Washington were among states that activated their National Guards to strengthen security. Texas closed its Capitol through to Inauguration Day.

In downtown Washington, officers arrested a Virginia man who presented an "unauthorised inauguration credential" at a Capitol Police checkpoint on Friday evening and was found to have a loaded handgun and more than 500 rounds of ammunition.

The man, identified as Wesley Allen Beeler, was driving a pickup truck with several firearm-related bumper stickers, including one that read, "If they come for your guns Give 'Em your bullets first," the court papers said. He was scheduled to appear in court on Saturday.

Responding to news of the arrest, Democrat representative Don Beyer of Virginia said the danger was real and the city was on edge.

"Anyone who can avoid the area around the Capitol and Mall this week should do so," Beyer wrote on Twitter.

The nationwide security scramble follows the deadly attack on the Capitol in Washington by extremists and Trump supporters.

Law enforcement officials have trained much of their focus on Sunday, when the anti-government "boogaloo" movement flagged plans to hold rallies in all 50 states.

In Michigan a fence was erected around the Capitol in Lansing and vehicles were mobilised from across the state to bolster security. The legislature cancelled meetings next week, citing concern over credible threats.

"We are prepared for the worst but we remain hopeful that those who choose to demonstrate at our Capitol do so peacefully," Michigan State Police Director Joe Gasper told a news conference on Friday.

Thousands of armed National Guard troops were in the streets in Washington in an unprecedented show of force after the assault on Capitol. Bridges into the city were to be closed, along with dozens of roadways.

Experts say that the capitals of battleground states such as Wisconsin, Michigan, Pennsylvania and Arizona are among those at most risk of violence. But even states not seen as likely flashpoints are taking precautions.

The alarm extended beyond government. The United Church of Christ, a Protestant denomination of more than 4900 churches, warned its 800,000 members there were reports "liberal" churches could be attacked in the coming week.

Australian Associated Press