America is currently undergoing the biggest prison strike in history, with prisoners in 40-50 prisons across 25 states participating in the organized action, it has been reported.

Using secret cell phones, social media and people on the outside, 'at least' 20,000 prisoners have downed tools in protest against free prison labor, The Intercept said.

'This is a call to action against slavery in America,' organizers on the Incarcerated Workers Organizing Committee (IWOC) said in a statement. 'Forty-five years after Attica, the waves of change are returning to America’s prisons.'

Strike: On September 9, the 45th anniversary of the Attica riot (pictured), prisoners across the US went on strike, saying their free or low-paid labor was a modern form of slavery

'This September we hope to coordinate and generalize these protests,' the statement continued, 'to build them into a single tidal shift that the American prison system cannot ignore or withstand.'

The strike began on September 9, the 45th anniversary of the Attica prison riot, in which prisoners demanded political rights and better living conditions.That riot saw 43 dead, 33 of whom were inmates.

'There are probably 20,000 prisoners on strike right now, at least, which is the biggest prison strike in history, but the information is really sketchy and spotty,' said Ben Turk of IWOC.

The current strike has seen prisoners complaining about various issues, including three-strike rules and lack of educational services.

But all are unified in their contempt for prison labor, which sees inmates working for as little as a few cents an hour - or even nothing, depending on the state.

Made legal under the 13th Amendment, which prohibits involuntary servitude 'except as a punishment for crime whereof the party shall have been duly convicted,' convict leasing allows private companies to hire prisoners for a pittance.

It's been used in businesses as diverse as military weapon production, mining and even - for a period in the 1990s - Victoria's Secret underwear.

Protest: Some 20,000 people are estimated to have joined in the strikes, which are said to be ongoing in various prisons (pictured: state police after riading the Attica prison)

Pastor Kenneth Glasgow, a former prisoner, said: 'People on the outside are not understanding they are being bamboozled.

'A lot of people are not realizing the value in what’s going on, they don’t realize that it’s slavery, that slavery still exists.'

Four facilities in Florida were affected when 'several hundred inmates' went on strike, according to the Florida Department of Corrections - but it added that the situation had been dealt with.

Michigan's Kinross Correctional Facility saw 400 prisoners staging a peaceful protest and 150 'ringleaders' being punished or transferred.

The IWOC Facebook page also quoted a number of sources saying strikes had occurred in North Carolina, South Carolina and Kansas.

Walk out: A strike at Holman prison in Alabama was reported - but a prisoner support group claims it led to staff walking out, leaving fewer than 15 officers to deal with 1,000 inmates

A strike of 45 people downing tools was also reported William C. Holman Correctional Facility in Alabama by Al.com.

The Free Alabama Movement, a prisoner-led organization that called for the mass strike, claimed that there had been a mass walk out by prison guards, leaving fewer than 15 officers to guard 1,000 inmates.

'Many areas of the prison, incl death row, are unattended,' it tweeted.