Extinction Rebellion is threatening legal action against counter-terrorism police for what it said was the illegal listing of the group as an extremist ideology in a guide designed to help stop terrorist violence.

The Guardian revealed on Friday that counter-terrorism police had placed the non-violent protest group on a list of extremist ideologies that should be reported to the authorities running the Prevent anti-radicalisation programme. Police now say that was an error.

Amnesty International condemned the decision on Saturday as criticism grew and questions remained about how Extinction Rebellion (XR) came to be included in the guide alongside neo-Nazi and terrorist groups.

The climate emergency campaign group was included in a 12-page document produced by Counter Terrorism Policing South East (CTPSE) titled “Safeguarding young people and adults from ideological extremism”.

XR has instructed lawyers.

Jules Carey, who acted for XR when it successfully struck down police protest bans in the courts last year, told the Guardian that the latest guidance was unlawful. “It is extraordinary that Counter Terrorism Policing South East have added Extinction Rebellion to the list of terrorist groups and extremist organisations that the Prevent strategy was set up to deal with.

“The guidance issued by the CTPSE is clearly unlawful. It constitutes an unlawful interference with human rights including free speech, right to assemble and enjoyment of a private life.

“The guidance is clearly designed to harm Extinction Rebellion and cast those who support the movement as domestic extremists. It is a glaring example of the sort of overzealous policing we have come to expect around protests. Being referred to Prevent could have long-lasting and life-changing consequences for a young school activist.”

After inquiries by the Guardian police said they were recalling the guidance and that XR had been included in error in the guide, which was distributed to police, government and teachers in November 2019.

Carey, of Bindmans solicitors, said: “It is not sufficient that CTPSE have sought to recall the guide from those that they shared it with. The advice in the guide … itself needs to be formally withdrawn and any decision taken to refer an activist of XR to Prevent needs to be urgently reversed.”

Kerry Moscogiuri, Amnesty International UK’s campaigns director, said the police guidance added to longstanding concerns about Prevent. “It’s deeply shocking that the police ever seriously considered classifying peaceful climate crisis protesters as extremists. To see that schoolchildren were effectively going to be profiled under these proposed measures, just deepens our shock.

“Given that children are potentially those who will be most affected by the climate emergency, it’s vital that they are able to speak out on these issues without this heavy-handed and entirely disproportionate police attention. This episode only adds to our existing concerns about Prevent, which is a highly dubious scheme sorely in need of a proper, independent and impartial review.”

Prevent is the the government’s anti-radicalisation programme. It is intended to spot people at risk of becoming involved in terrorism and divert them away from extremism.

Concerns have been raised that it impinges on freedom of thought and speech. Its supporters, which include the government and the police, say it is vital to stop the flow of recruits to Islamist and extreme rightwing violence, but it has been accepted that changes may be needed.

A review of Prevent that is due to report later this year is expected to be delayed after its original chair stepped down following criticism and a legal challenge.

The police document revealed by the Guardian said that issues to look out for included people who speak in “strong or emotive terms about environmental issues like climate change, ecology, species extinction, fracking, airport expansion or pollution”.

The guide advises readers to listen and look out for young people who “neglect to attend school” or “participate in planned school walkouts” – an allusion to the school strikes for the climate, a global movement of which the activist Greta Thunberg is a lead proponent. Thousands of UK pupils and millions worldwide walked out of school last year in protest at government inaction on the climate crisis.

The document also flags young people taking part in non-violent direct action, such as sit-down protests, banner drops or “writing environmentally themed graffiti”.

The disclosure that XR has been listed alongside proscribed groups such as National Action and al-Muhajiroun is being blamed on police in the south-east of England rather than a decision at the national level.

Clare Collier of the civil rights group Liberty said: “This is a startling example of how Prevent stifles free speech, labels dissent as dangerous and gives the police sweeping powers to monitor non-violent, non-criminal behaviour. The definition of extremism under Prevent is fundamentally flawed and so wide that thousands – including children – are swept up by it.

“Protest is fundamental to democracy and our ability to stand up to power. This year alone Extinction Rebellion, the school strikes and other protests have shown how valuable protest is for bringing about change. We corrode the right to protest at our peril.”

A spokesperson for the Muslim Council of Britain, which has long criticised how Prevent operates, said: “This case illustrates the serious flaws in the Prevent programme and a kneejerk reaction to create a list of undesirables, drawn up by divisive ideologues. The sooner a replacement independent reviewer can be found, the better.”

CTPSE would not answer questions about how XR came to be included.

On Friday DCS Kath Barnes, the head of CTPSE, said: “I would like to make it quite clear that we do not classify Extinction Rebellion as an extremist organisation. The inclusion of Extinction Rebellion in this document was an error of judgment and we will now be reviewing all of the contents as a result.

“It was produced by CTPSE to assist our statutory partners – including police forces and government organisations – in identifying people who may be vulnerable as a result of their links to some organisations.

“The document was designed for a very specific audience who understand the complexities of the safeguarding environment we work within and who have statutory duties under Prevent. We are in the process of confirming who it has been shared with and recalling it.

“We as counter-terrorism policing, along with our partners, have a responsibility to protect vulnerable people. Officers are trained to spot those who may be vulnerable, and the membership of an organisation that supports environmental or animal welfare issues alone would not be a trigger.”