Protesters gathered in major cities around the U.S. on Thursday over President TrumpDonald John TrumpChelsea Clinton announces birth of third child Ukrainian officials and Giuliani are sharing back-channel campaign information: report Trump attacks 'the Squad' as 'racist group of troublemakers' MORE's firing of Attorney General Jeff SessionsJefferson (Jeff) Beauregard SessionsThe Hill's 12:30 Report: Trump hits media over 'send her back' coverage House gears up for Mueller testimony Trump's no racist — he's an equal opportunity offender MORE, rallying in support of Robert MuellerRobert (Bob) Swan MuellerThis week: Mueller dominates chaotic week on Capitol Hill Top Republican considered Mueller subpoena to box in Democrats Kamala Harris says her Justice Dept would have 'no choice' but to prosecute Trump for obstruction MORE, the special counsel in charge of the investigation into Russian election meddling.

The hashtag "#ProtectMueller" was the top trending term on Twitter in the U.S. Thursday night as demonstrators gathered in Washington, D.C., New York and Boston, as well as other cities around the country.

A list of events hosted by the activist group Public Citizen listed dozens of events in smaller cities in nearly every state Thursday night as demonstrators protested what many thought to be an effort by the president to begin shutting down the Russia investigation.

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Sessions is set to be replaced by Matthew Whitaker as acting attorney general pending a permanent replacement to be confirmed by the Senate. Whitaker in the past has been a vocal opponent of the Mueller investigation.

Photos of events posted on Twitter showed hundreds of protesters gathered in various locations holding signs reading "Protect Mueller" and "Nobody is above the law."

Protests Thursday came hours after Arizona Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeFlake urges Republicans to condemn 'vile and offensive' Trump tweets Flake responds to Trump, Jimmy Carter barbs: 'We need to stop trying to disqualify each other' Jeff Flake responds to Trump's 'greener pastures' dig on former GOP lawmakers MORE (R), himself a frequent critic of the president, vowed to force a vote in the Senate on bill that would codify Justice Department regulations stating that only a senior DOJ official can fire the special counsel.

Trump has been critical of the Mueller investigation for months, deeming the probe which has ensnared several former members of his campaign in criminal charges as a "witch hunt" led by "angry Democrats."

Sessions had been expected to exit the Trump administration for months, and turned in his resignation to Trump on Wednesday just hours after Democrats retook the House in Tuesday's midterm elections.