Protesters gathered in major cities around the U.S. on Thursday over President TrumpDonald John TrumpVeterans groups demand end to shutdown: 'Get your act together' Brown launches tour in four early nominating states amid 2020 consideration Pence on border wall: Trump won't be ‘deterred’ by Dem ‘obstruction’ MORE's firing of Attorney General Jeff SessionsJefferson (Jeff) Beauregard Sessions5 takeaways from Barr’s testimony AG pick Barr emphasizes independence from Trump Hillicon Valley: Trump AG pick signals new scrutiny on tech giants | Wireless providers in new privacy storm | SEC brings charges in agency hack | Facebook to invest 0M in local news MORE, rallying in support of Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe 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 FlakeThe Hill's Morning Report — Trump eyes wall money options as shutdown hits 21 days Poll: Sanders most popular senator, Flake least CBS News in talks to hire Flake: report 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.