Donald Trump branded Joe Biden an "enemy of the state" on Saturday as he hit back at the US president's assertion that the Republican and his supporters are undermining American democracy, while slamming last month's FBI raid of his Florida home.

Making his first public appearance since the August 8 raid, Trump told a rally in Pennsylvania that the search was a "travesty of justice" and warned it would produce "a backlash the likes of which nobody has ever seen."

"There can be no more vivid example of the very real threats from American freedom than just a few weeks ago, you saw, when we witnessed one of the most shocking abuses of power by any administration in American history," Trump said, despite long-standing protocols by which the Justice Department and the FBI act independently of the White House.

He also hit back at Biden's speech this week in which the president said his predecessor and Republican supporters "represent an extremism that threatens the very foundations of our republic."

Speaking in Philadelphia, the cradle of US democracy, on Thursday, the president launched an assault on Republicans who embrace Trump's "Make America Great Again" ideology – and urged his own supporters to fight back in what he billed as a "battle for the Soul of the Nation."

Trump slammed it as the "most vicious, hateful and divisive speech ever delivered by an American president."

"He's an enemy of the state. You want to know the truth. The enemy of the state is him," Trump said.

"Republicans in the MAGA movement are not the ones trying to undermine our democracy," continued Trump, who has repeatedly claimed the 2020 presidential election, which he lost, was rigged; and whose party has made unfounded claims of voter fraud a central plank of their platform.

"We are the ones trying to save our democracy, very simple. The danger to democracy comes from the radical left, not from the right," Trump added.

He was appearing at the rally ahead of November's midterm elections, which could see Biden's Democrats lose control of both houses of Congress.