Protesters gathered in major cities around the U.S. on Thursday over President TrumpDonald John TrumpA better VA, with mental health services, is essential for America's veterans Pelosi, Nadler tangle on impeachment, contempt vote Trump arrives in Japan to kick off 4-day state visit MORE's firing of Attorney General Jeff SessionsJefferson (Jeff) Beauregard SessionsFive takeaways from Barr's new powers in 'spying' probe Amash: Some of Trump's actions 'were inherently corrupt' 'Persuadable' voters are key to the 2020 election — and the non-screaming news industry MORE, rallying in support of Robert MuellerRobert (Bob) Swan MuellerThe Hill's 12:30 Report: Trump orders more troops to Mideast amid Iran tensions Trump: Democrats just want Mueller to testify for a 'do-over' Graham: Mueller investigation a 'political rectal exam' 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 FlakeSen. Coons examines Amazon's privacy and data security practices for Alexa devices Oil companies join blitz for carbon tax The Hill's Morning Report - White House, Congress: Urgency of now around budget 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.