Sessions took the lead in imposing Trump's travel ban targeting a number of mostly Muslim nations, canceling the so-called Dreamer program that let the children of undocumented immigrants remain in the US and forcing the separation of migrant families as part of a border crossing crackdown.
Crowds also turned out at the White House and in Greensboro, North Carolina; Chattanooga, Tennessee; Las Vegas and many other places.
After months of bitter attacks on Sessions, Trump on Wednesday announced via Twitter that the former Alabama senator was being replaced. Whitaker was Sessions' chief of staff before Trump made him Sessions' interim replacement.
Kathleen Clark, a professor of legal ethics at Washington University in St. Louis, said Whitaker's previous comments about the Mueller probe raise questions about his impartiality and warrant his recusal. He wrote a year ago on CNN.com that Deputy Attorney General Rod Rosenstein should limit the scope of Mueller's investigation to stop him from delving into Trump's finances. In fact, Whitaker told CNN a year ago that a future attorney general could limit Mueller by reducing the budget "so low that his investigations grinds to nearly a halt".
Mueller's probe looks to establish whether Trump or his campaign actively colluded with them to steal the White House. Four former Trump campaign officials - including his onetime national security adviser Michael Flynn and campaign chairman Paul Manafort - are among those who have pleaded guilty.
Senate Intelligence Committee Vice Chair Mark Warner said Thursday on "Your World" that with President Trump's appointment of former Iowa federal prosecutor Matthew Whitaker as acting attorney general, the probability of the Mueller probe being tampered with will increase.
"When this happened yesterday, people started mobilizing very quickly", said Margaret Foley, the communications director for Nasty Women Get Shit Done PDX.
House Minority Leader Nancy Pelosi, in a separate statement, said US Congress must take action right away to protect Mueller's investigation against Trump's attempt to undermine it by firing Sessions.
Whitaker's argument, a common one among Trump supporters, completely misses the point of what obstruction of justice is.
"He's called it a lynch mob", said Bookbinder, head of the Citizens for Responsibility and Ethics, a Washington group. "You would always take that meeting, ' he said". They are going to go after him and you are seeing it already.
"I didn't speak to Matt Whitaker" about the Mueller probe, ' Trump said. Campaign efforts include ending the Iraq war and combatting gun violence by ending politicians' affiliation with the NRA, but since 2016 the organization says it has been "a pillar of the Resistance to Trump". But now, Mueller will need to get Whitaker's sign-off for those and other major steps - which he could withhold. It will also be Whitaker's decision whether to pass along evidence of any criminal wrongdoing by the president to Congress for any possible action.
Donald Trump told reporters on Friday that he has not discussed the Mueller investigation with his new acting attorney general, Matt Whitaker. In August 2017, Whitaker penned an op-ed in CNN saying he agrees with the president that Mueller has "come up to a red line that he is dangerously close to crossing". Whitaker accompanied the tweet with his own comment to his followers: "Worth a Read". The investigation is looking into United States allegations of Russian meddling in the election two years ago and any possible coordination between Russian and the Trump campaign, which the president denies.