Jacob Chansley, of Phoenix, Arizona, was photographed inside the Capitol shirtless, wearing a horned headdress and heavily tattooed.

He has been held without bond since his arrest shortly after the riot, and on Friday entered a guilty plea to obstructing an official proceeding.

While in detention, Chansley underwent mental examinations and was diagnosed by prison officials with transient schizophrenia, bipolar disorder, depression and anxiety.

Nearly 600 people have been arrested over the attack on the Capitol where Congress was meeting to certify Joe Biden's November victory over Trump. Earlier Trump had given a fiery speech falsely claiming his defeat was the result of fraud.

While the felony charge Chansley pleaded guilty to carries both a maximum 20-year prison term and a fine of up to $US250,000 ($A335,552), prosecutor Kimberly Paschall indicated the maximum sentence the government was likely to request would be much shorter.

Chansley had been a supporter of the QAnon conspiracy theory that casts Trump as a saviour figure and elite Democrats as a cabal of Satanist pedophiles and cannibals.

Although he did not get a pardon from Trump, Chansley's defence lawyer Albert Watkins said "there will always be a soft spot" for Trump in Chansley's heart.

At Friday's plea hearing, Watkins asked Judge Royce Lamberth to allow Chansley to be released from prison pending a sentencing hearing, scheduled for November 17. The judge said he would consider this request.

Watkins noted that prosecutors had acknowledged Chansley was "not a planner or organiser" of the riot.

Watkins later told reporters that Chansley had cooperated with January investigations and informed on a group he saw stealing classified materials from a Senate office.

Australian Associated Press