Former US president Donald Trump has called the appointment of a special counsel to investigate the presence of classified documents at his Florida estate a “horrendous abuse of power”.

US attorney general Merrick Garland on Friday named veteran prosecutor Jack Smith to oversee the Justice Department’s probe into the documents as well as key aspects of a separate investigation involving the January 6 2021 insurrection and efforts to undo the 2020 election.

Responding to the news that evening, Mr Trump called it an “appalling announcement today by the egregiously corrupt Biden administration and their weaponised Department of Justice”.

He said it was a “horrendous abuse of power” and “the latest in a long series of witch hunts,” and told the audience at an America First Policy Institute gala at his Mar-a-Lago estate that he had “done nothing wrong”.

The appointment, announced just three days after Mr Trump formally launched his 2024 candidacy, is a recognition of the unmistakable political implications of two investigations that involve not only a former president but also a current White House hopeful.

Mr Garland said that Mr Trump’s announcement of his presidential candidacy and President Joe Biden’s likely 2024 run were factors in his decision to appoint Mr Smith.

He said the appointment would allow prosecutors to continue their work “indisputably guided” only by the facts and the law.

“The Department of Justice has long recognised that in certain extraordinary cases it is in the public’s interest to appoint a special prosecutor to independently manage an investigation and prosecution,” Mr Garland said from the Justice Department’s podium.

“Based on recent developments, including the former president’s announcement that he is a candidate for president in the next election and the sitting president’s stated intention to be a candidate as well, I have concluded that it is in the public interest to appoint a special counsel.”

Though the appointment installs a new supervisor atop the probes – both of which are expected to accelerate now that the midterm elections are over – the special counsel will still report to Mr Garland, who has the ultimate say on whether to bring charges.

Mr Smith, a veteran prosecutor who led the Justice Department’s public integrity section in Washington and who later served as the acting chief federal prosecutor in Nashville, Tennessee, during the Obama administration, is set to begin his work “immediately”, Mr Garland said.

Mr Smith has also been the chief prosecutor for the special court in The Hague that is tasked with investigating international war crimes.

The Justice Department described Mr Smith as a registered independent, an effort to blunt any attack of perceived political bias.

Mr Trump is a Republican, and Mr Biden is a Democrat.