Liz Truss left the stage abruptly at an event to promote her own book after campaigners unfurled a banner behind her that was emblazoned with the phrase: “I crashed the economy” below a picture of a lettuce.

The former prime minister, who lasted 45 days in office, was in Suffolk on Tuesday discussing the US presidential election when the campaign group Led By Donkeys lowered its remote-controlled banner with a huge picture of a lettuce.

She said the prank was “not funny”. In a statement on X, Truss called Led By Donkeys “far-left activists” who used the stunt as a means to “intimidate people and suppress free speech”.

On Tuesday she expressed her horror at the “attacks on free speech in Britain and Europe”.

Truss was expressing her support for Donald Trump before she was pranked. “I support Trump and I want him to win,” she said before the audience applauded her. “It’s what I was saying a bit about incumbents. I think the average American is not doing well …

“I think it was Bill Clinton’s adviser who said: ‘It’s the economy stupid.’ So I think he [Trump] will, he probably will win. I’ve got a load of Trump questions, by the way.”

Moments later, as the host told her he had “no idea where that’s come from”, some members of the audience laughed, before she muttered: “That’s not funny,” ripping the microphone from her dress and walking off. She was applauded by some as she left.

The lettuce image and joke arose from the final days of Truss’s premiership, when the Daily Star launched a live stream of a lettuce to see whether Truss’s battle to survive in No 10 could last longer than a 60p iceberg lettuce from Tesco.

Truss was trying to promote her memoir, Ten Years to Save the West, when Led by Donkeys wrote on X: “We just dropped in on Liz Truss’s pro-Trump speaking tour with a remote-controlled lettuce banner. She didn’t find it funny.”

On X, Truss said: “What happened last night was not funny. Far-left activists disrupted the event, which then had to be stopped for security reasons. This is done to intimidate people and suppress free speech.

“I won’t stand for it. Would we see the same reaction if the activists were far-right?”

Truss has become vocal on US politics, last month addressing Republican supporters and urging them to learn lessons from her brief time in No 10. She said: “I’ve learned how powerful the unelected bureaucracy is. You have to win in November … you have to dismantle the leftist state … they are devious, they are ruthless and they are out to get you.”

The former prime minister has previously criticised the Daily Star’s lettuce joke, saying it was not “particularly funny”, noting in June: “I just think it’s puerile.”

She went on to criticise the British media, claiming it was “known throughout the world for being particularly vociferous” and it is not “particularly deferential to politicians”.

The banner stunt came shortly after Truss voiced her support for Elon Musk, who has claimed the UK has a two-tier policing system. She said on X: “I am appalled by the attacks on free speech in Britain and Europe. We can’t be truly free without free speech. Good for Elon Musk and X for standing up to these bullies.” Musk responded, thanking her for support.