All 50 US states and the District of Columbia (DC) are on alert for possible violent protests this weekend, ahead of President-elect Joe Biden's inauguration on Wednesday.
National Guard troops have been sent to Washington DC, to deter any repeat of last week's deadly riots.
The FBI has warned of possible armed marches by pro-Trump supporters at all 50 state capitols.
In DC, an armed man was arrested on Friday as security was ramped up.
Capitol Police confirmed on Saturday that a Virginia man carrying a "non-government issued credential" was stopped and arrested at a security checkpoint with at least one gun and 509 rounds of ammunition.
But the man, named Wesley Allen Beeler, was later released from custody and told the Washington Post that he he did not intend to bring a firearm into Washington, where he said had been working with a private security firm.
"I pulled up to a checkpoint after getting lost in DC because I'm a country boy," he said. "I showed them the inauguration badge that was given to me."
The arrest comes as authorities bolster security in Washington, where many streets - some miles from the Capitol - have been blocked off with concrete barriers and metal fences.
The National Mall, which is usually thronged with thousands of people for inaugurations, has been shut at the request of the Secret Service - the agency charged with protecting the president.
The Biden team had already asked Americans to avoid travelling to the nation's capital for the inauguration because of the Covid-19 pandemic. Local officials said people should watch the event remotely.
Sunday is expected to be a particular focus for protests, after posts on pro-Trump and far-right online networks called for armed demonstrations on that day.