(CNN) America's best known TV prosecutor came out of retirement Monday with a call to action in defense of embattled Deputy Attorney General Rod Rosenstein.

Speaking direct to camera in a new two-minute web video, Sam Waterston -- who played Manhattan district attorney Jack McCoy on NBC's hit crime series "Law & Order" for 16 years -- expressed concern about reports that President Donald Trump is considering firing Rosenstein and urged people to commit to peaceful protests if that happens.

"In America, there's a simple rule: no person is above the law," Waterston said in the video organized by TrumpIsNotAboveTheLaw.org , a coalition of more than two dozen groups including Common Cause, Public Citizen and MoveOn Civic Action. The video says it was paid for by the Equal Justice Society.

"Rod Rosenstein is the most important US prosecutor you've never heard of -- he's in the hot seat because he's the guy that can fire the special prosecutor Robert Mueller, who's investigating Russian interference with the 2016 election, and President Trump wants it to stop," Waterston added. "For a US president to fire a top government lawyer because he may find out about that president's own wrongdoing ... we Americans know that's just wrong."

CNN reported last week that Trump is considering dismissing Rosenstein as a way to curtail Mueller's probe -- a move that reached a boiling point last week following the FBI's raid on Trump's longtime attorney, Michael Cohen.

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