The most senior justice official in America laughed and repeated the words “lock her up” – an election campaign demand to jail Hillary Clinton – before an audience of high school students on Tuesday.

Speaking at a conservative conference in Washington, the attorney general, Jeff Sessions, said: “I like this bunch, I gotta tell you. You’re not going to be backing down. Go get ’em. Go get ’em.”

The gathering of high school students erupted with cries of “Lock her up, lock her up”, a reference to a trope still familiar at the president’s rallies.

Far from discouraging the chants, Sessions chuckled and repeated: “Lock her up,” adding with a smile: “I heard that a long time over the last campaign.”

The chant came about in 2016 as the FBI investigated Clinton’s use of a private email server while serving as secretary of state. Then FBI director James Comey concluded Clinton had been “extremely careless” but no charges should be filed.

Michael Flynn, Trump’s first national security adviser who pleaded guilty to lying to the FBI and is cooperating with special counsel Robert Mueller, memorably led the chant at the Republican National Convention.

In one presidential debate, Trump told Clinton that if he was in charge of the nation’s laws, “you’d be in jail”, a threat that provoked widespread comparisons to leaders of authoritarian regimes who threaten to imprison opponents.

On Tuesday, Sessions’ flippant response met with fierce criticism. Richard Painter, a Bush-era White House chief ethics counsel now running for the Senate as a Democrat in Minnesota, tweeted: “This is the face of fascism. Either we remove these people from office – every one of them – or we lose our representative democracy. Impeach now!”

Scott Dworkin, co-founder of the Democratic Coalition, an anti-Trump grassroots group, posted: “Jeff Sessions just led a chant of ‘lock her up’ at a high school leadership summit. The Attorney General. In front of high schoolers. This is absolutely unreal and terrifying. What a coward.”

Trump has expressed frustration with Sessions for not looking into Clinton’s deleted emails and publicly criticised him for being “very weak” on her supposed “crimes”. Sessions, a former senator from Alabama, assigned a federal prosecutor to look into various matters around Clinton but resisted pressure to appoint a special counsel.

On Tuesday, Sessions was speaking at Turning Point USA’s High School Leadership Summit at George Washington University, where speakers included the United Nations ambassador, Nikki Haley, former Ukip leader Nigel Farage and former White House press secretary Sean Spicer.

Sessions’ speech was about justice department efforts to protect free speech on university campuses. He mocked students he said held a “cry-in”, used “therapy dogs” and were offered “Play-Doh and colouring books” after Trump’s victory.

“Rather than moulding a generation of mature and well-informed adults,” he said, to cheers, “some schools are doing everything they can to create a generation of sanctimonious, sensitive, supercilious snowflakes. We’re not going to have it!”