On 15 April of this year, seven inmates at the Lee Correctional Facility in South Carolina were stabbed and beaten to death in the deadliest prison riot in at least a quarter century. Prison officials took hours to stop the melee, inmates claimed, and later stacked the bodies of the dead in piles outside the prison yard. One inmate said he bled for hours before correctional staff came to his aide.

Officials claimed they only waited to intervene until they had enough resources to ensure the safety of inmates, and later blamed the riot on gang warfare and competition over mobile phones. But prisoners have alleged in legal filings that the violence was related to inhumane conditions at the state’s largest maximum-security prison.

Four months later, prisoners across the country are hoping to thrust these conditions into the national spotlight, in what is expected to be the largest prison strike in US history.

Inmates in at least 17 different states have signed on to participate in the national prison strike, which starts on 21 August. More than 100 outside groups have endorsed the strike, including chapters of Black Lives Matter, the Democratic Socialists of America and activists groups at Harvard and Princeton.

The protest was originally planned for 2019, as a sequel to the first-ever nationwide prison strike in 2016. But according to organisers, the violence in South Carolina convinced them that the time to act was now.

“It made it abundantly clear to the leadership, who live in facilities like Lee, that they had to mobilise now,” said Brooke Terpstra, a spokesperson for the Incarcerated Workers Organising Committee (IWOC), an outside group helping to publicise the strike.

The prisoners have released a list of 10 demands, ranging from improvements in prison conditions to changes in the legal system. According to organisers, the diverse set of demands reflects the equally diverse groups of voices who helped plan the protest.

The key themes, however, are clear: More rights – and more money – for incarcerated workers; increased access to rehabilitation programmes, and an end to sentencing practices that many feel target racial minorities.

“Prisoners understand they are being treated as animals,” the organisers said in a statement earlier this month. “We know that our conditions are causing physical harm and deaths that could be avoided if prison policy makers actually gave a damn … For some of us it's as if we are already dead. So what do we have to lose?”

The strike participants plan to take whatever actions they can in order to slow, or even shut down, the operations at their facilities. Those with jobs will refuse to show up for work; those with money will refuse to spend it at commissaries. In some prisons, inmates will engage in “peaceful sit-ins” by simply refusing to move. Prisoners without a job, money, or freedom of movement can simply refuse to eat.

It’s unclear exactly how many people will participate in the strike, as communication with the outside world is slow and often censored. But according to organisers, the protest is already larger than its 2016 precursor, which affected an estimated 60,000 to 70,000 inmates, either through participation in the strike or lock-downs in their units.

UK prison conditions: in pictures Show all 8 1 /8 UK prison conditions: in pictures UK prison conditions: in pictures A cell covered in graffiti at HMP Liverpool. PA UK prison conditions: in pictures HMP Liverpool has some of the worst conditions inspectors have seen. HM Inspectorate of Prisons/PA UK prison conditions: in pictures A broken window in a cell at HMP Liverpool HM Inspectorate of Prisons/PA Wire UK prison conditions: in pictures A shower unit with protruding electric cable at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures Litter at HMP Liverpool HM Inspectorate of Prisons/PA Wire UK prison conditions: in pictures A wall damaged by damp at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures A pool table at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures A sign for HMP Liverpool, where drones are seized at a rate of more than one a week. The prisons watchdog flagged up the impact of the remote-controlled flying devices at HMP Liverpool. PA

What makes the strike complicated, however, is that it is technically illegal. Inmates in the US are not allowed to organise, form unions, or even congregate in large groups. Several of the 2016 strike organisers were transferred to other prisons or placed in solitary confinement before the protest even began. This year, the South Carolina Department of Corrections has said they are keeping tabs on any possible strike activities.

“The agency will take the necessary actions to ensure the prisons are safe to operate and there are no threats to safety,” spokesman Dexter Lee told local newspaper The State.

But that hasn’t stopped prisoners from organising anyway. Mr Terpstra emphasised that while the IWOC is helping to publicise the strike, the action itself was planned by people on the inside. In fact, that was one of the goals of the protest: To give prisoners a voice of their own, instead of relying on outside organisations to do it for them.

Crucial in this effort is the spread of contraband cell phones within the prison system. Friends, family members, and even guards often smuggle phones into the prisons, which can then be used to call or text other inmates. Mr Terpstra said this year’s organisers even held conference calls from inside their cells. More importantly, the phones allow them access to social media, where they can broadcast their plans and demands on platforms like Twitter and Facebook.

Prisoners also communicate the old-fashioned way, through letters and visitations with outside advocates. Inter-prison transfers, often used as a way to separate organisers, can actually act in the inmates favour: Organisers transferred to other prisoners can spread the word about the protest there, as well.

Kevin Steele, a former New York inmate who helped organise the 2016 strike, said the process of planning the strike was cathartic in and of itself.

"You find that people who have the same mindset as you, and you just sit there and talk and build on things, and eventually find something worth fighting for,” he told The Independent.

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Mr Steele was released in January of this year on appeal, after serving eight years of his 10-year sentence. He has been working with IWOC ever since, helping to organise prisoners like himself, who want to see a change in prison conditions for future generations.

He is currently in touch with one inmate, a transgender woman, who is interested in joining the strike. She is being held in a men’s prison against her will, despite legally changing her name and starting hormone therapy.

Mr Steele said it was inmates like this – who want to join the movement but don’t know where to start – that made him commit to organising even while he was still behind bars.

“I just felt the need that, If I ever get the opportunity to go home, I'm going to continue to fight for the people who don't have a voice,” he said.

The strike participants know that they are up against a powerful adversary, and making some difficult demands. According to Mr Terpstra, they know that their demands are unlikely to be met over the course of a few weeks – or even at all. The point, he said, is to get people talking about them.

"They essentially felt compelled to put their voices out there,” he said. “They essentially said, life can't continue this way."