The US attorney general, William Barr, said on Saturday Donald Trump had fired Geoffrey Berman, the US attorney for the southern district of New York who has overseen investigations and prosecutions of key Trump allies including Rudy Giuliani and Michael Cohen.

Berman later confirmed he had now stepped down and that his deputy Audrey Strauss would take his place.

“I could leave the district in no better hands than Audrey’s,” Berman said in a statement. He added that he was sure Strauss would continue the district’s “tradition of integrity and independence”.

The move brings to a likely end a bizarre dispute between the attorney general and Berman, after Berman earlier refused to confirm his resignation from the prestigious role, following Barr’s announcement in a surprise statement on Friday night. That refusal prompted Barr to announce he had asked Trump to fire Berman – which the president apparently promptly did

Before Barr moved to fire Berman, observers widely noted that though the attorney general did not have the authority to do so, the president did.

Characteristically, Trump appeared to muddy already murky waters. Leaving the White House for his rally in Tulsa, Oklahoma, he told reporters the firing was “all up to the attorney general. Attorney General Barr is working on that. That’s his department, not my department. But we have a very capable attorney general, so that’s really up to him. I’m not involved.”

Steve Vladeck, a professor of law at the University of Texas, said on Twitter that more important than questions about legal authority was “the much bigger question is why this happened”.

Barr had been widely accused of undermining Department of Justice independence even before he moved against Berman.

In a surprise statement released on Friday night, the attorney general said Trump intended to nominate Jay Clayton – the chair of the US Securities and Exchange Commission but with little experience as a federal prosecutor – as US attorney. Barr said then the US attorney in New Jersey, Craig Carpenito, would be acting US attorney until Clayton could be confirmed by the Senate.

In response, Berman said he had not known of the move until Barr’s statement was issued.

“I will step down when a presidentially appointed nominee is confirmed by the Senate,” he said. “Until then, our investigations will move forward without delay or interruption.

“I cherish every day that I work with the men and women of this office to pursue justice without fear or favor – and intend to ensure that this office’s important cases continue unimpeded.”

On Saturday, Berman showed up to work in Manhattan.

“I’m just here to do my job,” he said.

He did not immediately comment after Barr moved to have him fired.

Barr accused Berman of having “chosen public spectacle over public service”.

“Your statement also wrongly implies that your continued tenure in the office is necessary to ensure that cases now pending in the southern district of New York are handled appropriately,” he wrote. “This is obviously false. I fully expect that the office will continue to handle all cases in the normal course.”

Berman was not confirmed by the Senate but appointed by district judges until a Senate-confirmed nominee was found, according to law and after Trump fired the previous US attorney, Preet Bharara, an Obama appointee.

In a tweet on Saturday, Bharara mocked Barr’s letter as a “wrinkle”.

“The president just said that he was not involved in the matter of Geoff Berman in SDNY,” he wrote. “That means that though both Trump and Barr lie, at least one of them is lying about who is firing Berman.”

Berman has overseen investigations into Giuliani’s business dealings – no wrongdoing has been formally alleged – and campaign finance violations which contributed to Cohen going to jail.

The SDNY is also overseeing the prosecution of Lev Parnas and Igor Fruman, two associates of Giuliani, on campaign finance charges. Parnas has turned against Trump.

On Saturday ABC News released a section of a forthcoming interview in which John Bolton, the former national security adviser whose tell-all book will be published on Tuesday despite Trump’s attempt to stop it, said the president’s suggested intervention in an SDNY case involving a Turkish bank “did feel like obstruction of justice to me”.

The southern district is an immensely powerful office, often charged with prosecuting financial, organised crime and terror-related cases. Berman oversaw the case of Jeffrey Epstein, the disgraced financier who counted Trump among influential friends and who killed himself in custody in New York last year.

Among Democrats, Val Demings, a US representative from Florida and a possible pick as Joe Biden’s presidential running mate, promised Congress would establish the reason for Berman’s dismissal.

“What was Mr Berman investigating that the president and his fixer are so determined to stop?” she said in a tweet. “We are going to find out.”

That followed another tweet in which she said: “The president has doubled down on his fixer’s obstruction of investigations into him and his allies. It is clear that nothing will restrain his corruption. No one is above the law. The American people will have their say.”

Letitia James, the New York attorney general, issued a hard-hitting statement.

“I am deeply concerned about the sudden removal of the US attorney for the southern district of New York, especially given serious questions about who gave the order and the timing of the firing,” James said.

“I hope the Department of Justice heeds its own advice and puts public service over public spectacle. Ongoing investigations must not be interfered with, period. There is still much work to be done and it must be done independently. Americans deserve leaders who are committed to justice.”