Washington is on high alert ahead of a rally in support of the pro-Trump rioters who ransacked the US Capitol, with security forces better prepared to avoid a repeat of the mayhem seen on 6 January.

A black fence that surrounded the complex for six months after the deadly insurrection has been put back up for the weekend, along with surveillance cameras, although police don't have indications of a specific plot associated with Saturday's "Justice for J6" rally.

Homeland Security officials nevertheless warned about the potential for violence, according to an intelligence briefing obtained by US media, with a counter-rally scheduled to take place nearby.

"We are aware of a small number of recent online threats of violence referencing the planned rally, including online discussions encouraging violence the day before the rally," the brief from the DHS Office of Intelligence and Analysis says, according to CNN.

Protestors surrounded the Capitol Building in January (File picture)

Government officials are expecting around 700 protesters. Capitol Police were due to give details of their preparations for the demonstration at 1pm news conference.

"The USCP has asked the Department of Defense for the ability to receive National Guard support should the need arise on 18 September," the force said in a statement earlier this week.

Members of Congress will not be in the building, with many still on summer recess and not back in town until next week.

Organiser Look Ahead America is planning similar rallies across the country in the coming weeks but says the gatherings are to support people being prosecuted for non-violent offenses.

They have appealed to attendees to be respectful to law enforcement officers and refrain from bringing Trump banners.

"We have composed a draft resolution a state legislature can pass to inform US Senators and Representative to oppose the tyrannical and inhumane treatment of the 6 January political prisoners who have been targeted by the Department of Justice and the FBI," executive director Matt Braynard said in a statement.

Thousands of Donald Trump's supporters, many associated with ultra-nationalist and white supremacist groups, stormed the US legislature eight months ago in an effort to overturn President Joe Biden's election victory.

They had been egged on by Mr Trump, whose fiery speech earlier that day falsely claiming election fraud was the culmination of months of baseless claims about a contest he lost fairly to Mr Biden.

"Our hearts and minds are with the people being persecuted so unfairly relating to the January 6th protest concerning the Rigged Presidential Election," Mr Trump said in a statement yesterday.

More than 600 people have been charged over the riot, according to reports, and dozens have pleaded guilty.

The majority are facing low-level misdemeanour charges but rioters admitting felonies face stiff prison sentences.