Sessions writes that it was "an incredible honor to work with people who day after day and year after year consistently exceeded expectations".
That drew criticism from Democrats because Whitaker, who would now take over responsibility for overseeing Mueller and his investigation, has been critical of the Mueller probe, saying it should be scaled back.
The protest campaign claims on its website that with Sessions ousted and acting Attorney General Matt Whitaker replacing Rod Rosenstein in overseeing the Mueller investigation, Trump "undercut the independence of the investigation" because "Whitaker has publicly outlined strategies to stifle the investigation and can not be allowed to remain in charge of it".
Acting Attorney General Matt Whitaker has no intention of recusing himself from overseeing the special counsel probe of Russian interference in the 2016 election, according to people close to him who added they do not believe he would approve any subpoena of President Trump as part of that investigation.
President Trump fired Jeff Sessions from his attorney general post yesterday.
Gonzales said if a decision is made to curtail or end the Mueller probe, that shouldn't be made by the acting attorney general, but someone within the normal lines of the chain of authority, either the deputy attorney general or the solicitor general. He questioned Deputy Attorney General Rod J. Rosenstein's ability to give Mueller such wide latitude and wanted to explore the bounds of what Mueller was examining, though Rosenstein kept Session's office "walled off" from the matter, this person said.
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Mueller is investigating Russia's alleged interference in the 2016 election and any collusion by Trump's campaign.
A Kremlin spokesman, Dmitry Peskov, on Thursday called Mueller's investigation a "headache" for USA authorities but said it "has nothing to do with us", and he declined to comment on Sessions' departure.
Many voiced concerns Wednesday that Whitaker may have potential conflicts of interest, especially in light of his attacks on the investigation.
Over the course of his investigation thus far, Mueller and his team of prosecutors indicted 32 individuals and three Russian businesses on charges ranging from computer hacking to conspiracy and financial crimes.
Kathleen Clark, a professor of legal ethics at Washington University in St Louis, said Mr Whitaker's previous comments about the Mueller probe raise questions about his impartiality and warrant his recusal.
Whitaker is a former US attorney from Iowa who founded a law firm with other Republican Party activists. Nadler and other Democrats on the committee suggested the shakeup appears to be an effort to alter oversight of special counsel Robert Mueller's probe in a way that favors the president.