The former FBI director has been working with a team of prosecutors and agents for the previous year and a half to investigate whether the Trump campaign illegally coordinated with Russian Federation to tip the 2016 election, and whether Trump tried to obstruct that investigation.
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.
Republican Senator Lamar Alexander of Tennessee said Mueller's probe will continue to its end "because no new attorney general can be can be confirmed who will stop that investigation".
A Justice Department spokeswoman, asked if Mr Whitaker would now oversee Mueller, replied: "The acting attorney general is in charge of all matters under the purview of the Department of Justice". He accused it of overreach. Trump, who denies any collusion, has long complained about the probe, calling it a witch hunt.
Those include an opinion piece on CNN.com in which he said Mueller would be going too far, and straying beyond his mandate, if he were to investigate Trump's family finances.
Protesters held signs reading: "Putin/Trump Make Russia Great Again", "Just when you thought things couldn't get any worse", and "Only guilty men fear the truth".
Sessions, who likely suspected his ouster was imminent, was spotted by reporters giving some of his grandchildren a tour of the White House over the weekend.
Trump saw the move as a betrayal and frequently and publicly criticised Sessions for making the decision to recuse himself.
Man Fire Drives Through Camp Fire Flames in Northern California
When he arrived there, patients were out in the front of the emergency room, where the roof had caught fire. California has experienced debilitating fires of unprecedented regularity in the past few years.
The president deflected questions about Sessions' expected departure at a White House news conference Wednesday. One associate, Jerome Corsi, said in a video recording Monday that he's "been involved in a really constant basis" for two months with Mueller's investigation.
"Whitaker has made clear that he can't be an impartial overseer of this critical investigation".
Whitaker now becomes Mueller's new boss and judging by past statements he will be sympathetic towards Trump.
How many people will show up remains to be seen.
Several of the groups involved in Thursday evening's protests have always been planning what they call "rapid response" protests if Trump tried to fire Mueller or Deputy Attorney General Rod Rosenstein, who had been overseeing Mueller's investigation.
David Bosel, a retired political scientist, described Trump's attempts to scupper the Mueller investigation as "dictatorial" and accused most Republicans in Congress of helping Trump tilt the United States towards "fascism".
Last year, while serving as Sessions' chief of staff, Whitaker shared an article urging Trump's attorneys not to cooperate with the "Mueller lynch mob". Trump had repeatedly and openly attacked Sessions for recusing himself from the Mueller investigation. Whitaker did, however, believe that Sessions had no choice but to recuse himself from the matter, the person said.
She went on to argue that, "given his record of threats to undermine and weaken the Russian Federation investigation", Mr Whitaker should follow in Mr Sessions' footsteps and recuse himself. He called Trump's recent actions "a rerun of the 'Saturday Night Massacre, ' " a reference to the October 20, 1973 resignations of the attorney general and deputy attorney general after each refused an order by President Richard Nixon to fire Watergate special prosecutor Archibald Cox.