US Attorney General Jeff Sessions has endured more than a year of criticism from Donald Trump over the Mueller inquiry. Whitaker plans to assume oversight of Mueller's investigation from Deputy Attorney General Rod Rosenstein.
Mr Sessions removed himself from the probe after Democrats accused him of failing to disclose contacts he had had with the Russian ambassador as a senior adviser to Mr Trump's campaign.
The man who will serve at least temporarily as the nation's top law enforcement official is Matthew G. Whitaker.
When asked if he would fire Mueller, as some critics have feared, Trump said, "I could have ended it any time I wanted, [but] I didn't".
Warner criticized Trump for not appointing a bureaucrat who previously got senatorial confirmation - naming Rosenstein and Solicitor General Noel Francisco. This is the much-anticipated finishing touch on Mueller's Russia Investigation mandate.
Trump said he will nominate a permanent successor for Sessions at a later date.
The two people close to Whitaker also said they strongly believe he would not approve any request from special counsel Robert S. Mueller III to subpoena the president.
Separately, Justice Department prosecutors in NY secured a guilty plea from Trump's former personal lawyer, Michael Cohen, who said the president directed him to arrange hush money payments before the 2016 election to two women who said they had sex with Trump.
"Trump's actions are being compared with those of U.S. President Richard Nixon in one of the most controversial episodes of the Watergate scandal, the Saturday Night Massacre", said organizers.
Ginsburg, 85, hospitalized after fracturing 3 ribs in fall
Since taking office, President Donald Trump has appointed two conservative justices to the court, shifting it to the right. Broken ribs typically heal on their own in six weeks to a month, and patients are advised to limit strenuous activity.
In Sessions' place, Trump has announced Matthew Whitaker as the acting attorney general.
St. Petersburg Mayor Rick Kriseman told the crowd that firing Sessions already amounted to the "biggest constitutional crisis we have seen since Watergate".
Whitaker now becomes Mueller's new boss and judging by past statements he will be sympathetic towards Trump. I've known Jeff for a long time.
"Committees of the United States Congress are conducting investigations parallel to those of the special counsel's office, and preservation of records is critical to ensure that we are able to do our work without interference or delay", the Democrats wrote. And though Democrats have called for him to recuse himself because of his past comments, the case is less clear-cut than it was for Sessions - who stepped aside because of his prominent role in Trump's campaign.
Thanking Sessions for his service, Romney said that it was "imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded". He had frequently publicly castigated Sessions for recusing himself a year ago from the case.
The broadsides escalated in recent months, with Trump telling an interviewer that Sessions "never had control" of the Justice Department.
"It should end because it's very bad for our country", he said.
He also backed Trump's ban on the entry into America of people from several Muslim-majority countries.
The announcement, which came in a tweet, comes after months of speculation that Sessions could be on his way out after recusing himself from the investigation into Russian interference in the 2016 presidential elections.