Environmental protest group Extinction Rebellion has said it is sorry for the wording of its online prison guide, after a backlash from civil rights activists.
More than 1,100 protesters were arrested in April, when the group shut down parts of London for 10 days, and future actions are planned.
The guide, which has now been deleted from its website, advised that the risk of violence inside prison was very low because "most prison officers are black and do not wish to give you a hard time".
It also suggested people should use their time in prison to "practise yoga" and "learn from their experience".
A spokesperson for Extinction Rebellion commented: "We are aware that the experience of the contributors is not reflective of many who have experienced the criminal justice system. We have now taken it down and are in the process of reviewing it.
"We would like to apologise for the wording used in this document"
'Not a yoga retreat'
Civil rights activists warned that the advice was "inaccurate and misleading" and added that violence and discrimination are commonplace in prisons.
In England and Wales, assaults in prisons are currently at a record high and in 2018, there were 325 deaths in prison custody - up 10% on the previous year.
And contradicting Extinction Rebellion's claim, 94% of prison officers in England and Wales were white as of March 2018.
"Prison is not - and I cannot stress this enough - a yoga retreat," commented lawyer and anti-prison activist Eda Seyhan.
"Prisons are dangerous and oppressive institutions where disproportionate numbers of black and brown men are locked away from the rest of society.
"I've spent enough time in police stations and prisons to know that violence and discrimination are rife - no amount of yoga or meditation can change that," Ms Seyhan added.
Extinction Rebellion aims to cause non-violent disruption resulting in arrest in order to highlight the risk posed by climate change.
Three protesters have been charged and put on remand for a month for climbing on top of a train in Canary Wharf station.
'Moving and maturing experience'
The prison guide advised people to take books with "plenty of good tips for yoga and meditation" and to "structure the space with time for sitting, yoga, breathing exercises, journalising, creative art".
It continued: "If you get solitary [confinement], there's plenty of time for meditation. Lastly you can take as many naps as you want!"
A disclaimer in the prison guide stated "we cannot be responsible if matters turn out differently to what we all expect".
A Twitter user wrote: "The prison guide is really rather responsible," and added that she believes Extinction Rebellion has "many good points".
Another person tweeted: "If your campaigning intrinsically involves the possibility of arrest, there must be safeguards in place to support people who put themselves in that vulnerable position.
"The glamourisation of arrest and imprisonment is so problematic."
The criticism of the guide comes as others, including online magazine Gal-Dem, have suggested that Extinction Rebellion "whitewashes" climate change and alienates certain people who may have more to lose by being arrested than others.
This includes immigrants, whose visa status could be affected by a criminal record, or black people, who 2018 figures suggested, are more likely to have force used against them by the Metropolitan Police.