Claims by a senior official in the Trump administration that British police were making thousands of “freedom of speech” arrests have been rejected by the UK government.

Sarah B Rogers, who has become the public face of the US state department’s hostility to European liberal democracies, was accused by MPs of echoing far-right memes and conspiracy theories during a speech at an international rightwing conference in London. She also referenced the death of Henry Nowak and a recent incident in which a child was thrown into a zoo’s crocodile pit.

Rogers, who has publicly attacked policies on hate speech and immigration by US allies and promoted far-right parties abroad, centred her speech on the notion of “Da Yookay” – a viral meme heavily associated with the online far right.

Speaking at the Alliance for Responsible Citizenship (ARC), which was also addressed this week by Nigel Farage and Kemi Badenoch, Rogers listed what she said were examples of the Britain that people saw online. “In ‘Da Yookay’, you can be remanded without bail for an inflammatory tweet, while a psychopath who seizes a three-year-old and feeds him to crocodiles walks free.

“In ‘Da Yookay’ the moral sense of jurors won’t save you, because jury trials for speech crimes are abolished. In ‘Da Yookay’, a girl can escape from a rape gang, flag down a police constable and discover the cop is in league with the rapists.

“In ‘Da Yookay’ you get a free car for pretending to be disabled. In ‘Da Yookay’ cops defer to a murderer who calls his victim racist. Then they handcuff you as you bleed to death if you’re white.”

Rogers told the audience that she was not there to tell them “as your minders do, that it’s all misinformation”.

Rogers is undersecretary of state for public diplomacy, a US state department role that was created in 1999 to strengthen relationships between the US and foreign publics.

Her intervention at ARC marks the most explicit criticism of the UK government by a US official on British soil. She also claimed: “Some people look at Britain’s thousands of speech arrests per year and see only tyranny.”

A UK government spokesperson said on Friday: “Our world-renowned justice system operates without fear or favour to protect all our citizens, and we completely reject this characterisation.”

Max Wilkinson, the Liberal Democrats home affairs spokesperson, said repeated attempts by Trump officials to undermine the UK’s democracy and justice system were out of hand and ministers should contact counterparts in the US. “Echoing bizarre online conspiracy theories about the UK is something we might expect from a hostile state rather than a Nato ally,” he added.

Citing gun violence in the US and deportations of children, the Labour MP Stella Creasy, also hit back, saying figures like Rogers “should spend less time reading Twitter conspiracy theories about the UK and more time fixing their own problems”.

Keir Starmer suggested earlier this month that the US was trying to interfere in British democracy after JD Vance, the US vice-president, blamed the murder of the British teenager Henry Nowak on mass immigration.

More than 4,000 delegates from 85 countries attended the three-day conference, which has emerged as a force shaping policies on the right both in the Britain and abroad. It is headed by the influential Conservative peer Philippa Stroud, a former adviser to Iain Duncan-Smith and, with him, was one of the architects of the universal credit overhaul of welfare.

ARC’s advisory board includes the Reform MP Danny Kruger and James Orr, a Cambridge theologian who is a senior advisor to Nigel Farage. Other themes promoted by ARC include calls for action to tackle a “demographic decline” in the west and encourage people to have more children.

Kruger told delegates in a keynote speech that Britain’s cultural and political crisis was a battle to defend what he called the “English settlement” – a civilisational inheritance rooted in scripture, national sovereignty and the rule of law.

Appearing a day after Badenoch, the Conservative party leader, Farage made an explicit pitch for support at the gathering in west London, likening “family breakdown” to “community breakdown” as populations grew more diverse.

Attenders over the past week have included delegates from members of the European far-right groups: Alternative für Deutschland, Vlaams Belang from Belgium, Spain’s Vox and the Netherlands’ Party for Freedom.

Also on stage as an official participant in one event was Carl Benjamin, a provocative British rightwing political commentator known by his online pseudonym Sargon of Akkad and for speculating about whether he would rape the Labour MP Jess Phillips.