It was a defining moment in British political history, paving the way in the long struggle for democratic representation of the disenfranchised working classes.

Now, 200 years on from the Peterloo massacre in which peaceful protesters were cut down by sabre-wielding cavalry, a hastily scribbled note has been unearthed to reveal what could be the first account of the bloodshed.

Penned on a scrap of paper by a magistrate who watched the violence unfold, the message was immediately dispatched from the scene in Manchester to Whitehall by horse-drawn carriage, arriving more than 24 hours later.

It is among a treasure trove of documents released by the National Archives to mark the bicentenary of the slaughter, in which 18 people were killed and more than 650 injured when cavalry charged at pro-democracy protesters gathered at St Peter’s Field.

Quick guide What was the Peterloo massacre? Show Hide What was the Peterloo Massacre and how many were killed? On 16 August 1819, up to 60,000 working class people from the towns and villages of what is now Greater Manchester marched to St Peters Fields in central Manchester to demand political representation. Their peaceful protest turned bloody when Manchester magistrates ordered Yeoman – a private militia paid for by rich locals – to storm the crowd with sabres. Most historians agree that 14 people were definitely killed in the massacre – 15 if you include the unborn child of Elizabeth Gaunt, killed in the womb after she was beaten by constables in custody. A further three named people are believed to have either been stabbed or trampled to death. Why is it called Peterloo? The name was first coined five days after the massacre by James Wroe, editor of the Manchester Observer, the city’s first radical newspaper (no relation to the Observer of today). According to historian Robert Poole, Peterloo was “a bitter pun, comparing the cowardly attacks by the Yeomanry and soldiers on unarmed civilians to the brutality suffered at Waterloo.” What did the protesters want? They wanted political reform. The years leading up to Peterloo had been tough for working class people and they wanted a voice in parliament to put their needs and wants on the political agenda, inspired by the French Revolution across the Channel. Machines had begun to take jobs in the lucrative cotton industry but periodic trade slumps closed factories at short notice, putting workers out on the street. The Napoleonic Wars, which ended in 1815 with the Battle of Waterloo, had taken a heavy toll on the nation’s finances, and 350,000 ex-servicemen returned home needing jobs and food. Yet those in power seemed more interested in lining their own pockets than helping the poor. At that point, only the richest landowners could vote, and large swathes of the country were not represented in Westminster. Manchester and Salford, which then had a population of 150,000, had no MP, yet Oxford and Cambridge Universities had their own representation. At the time the extension of the vote to all men, let alone women, was actively opposed by many who thought the vote should be restricted to those of influence and means. Why is Peterloo important? It paved the way for parliamentary democracy and particularly the Great Reform Act of 1832 which created new parliamentary seats, particularly in the industrial towns of the north of England. It also led to the establishment two years later of the Manchester Guardian by John Edward Taylor, a 28-year-old English journalist who was present at the massacre and saw how the “establishment” media sought to discredit the protesters. Helen Pidd, North of England editor Photograph: Rischgitz/Hulton Archive

The massacre – later dubbed “Peterloo” in reference to the Battle of Waterloo which took place four years earlier – was recently brought to the big screen in a feature-length Mike Leigh adaptation.

Henry Hunt, a radical reformer who was later jailed, spoke at the protest which was calling for parliamentary reform at a time when large swathes of the country were unrepresented and only wealthy landowners could vote.

Dressed in their Sunday best, tens of thousands of demonstrators flocked from miles around, carrying banners and flags. Unusually for the time, the crowd included a sizeable number of women, clad in white to symbolise their virtue as they joined the fight for suffrage.

The newly released documents, which included correspondence from the government and protesters, give an intimate portrayal of events from both sides.

The first account of the atrocity was recorded by Haigh Allen, a young magistrate from Huddersfield who was in Manchester to monitor events, barely an hour after the massacre on 16 August 1819.

“Gents,” he wrote at 2.30pm. “The meeting took place at 1 o’clock. Hunt in the chair with 16 flags and 7 caps of Liberty hoisted up amongst upwards of 60,000 people, the cavalry has just broke in upon them, the flags are taken, Hunt and his party secured, several lives are lost and a number wounded. The cavalry are now securing the streets in all directions, ½ past 2 o’clock, yours H. Allen.”

Facebook Twitter Pinterest Haigh Allen’s account of the Peterloo massacre. Photograph: National Archives

The violence was triggered after magistrates ordered the Manchester and Salford Yeomanry, a volunteer cavalry regiment who some accused of being drunk, to arrest Hunt and other organisers, fearing they had revolutionary intent.

Galloping to detain Hunt, one rider knocked down a woman and killed her two-year-old child who was thrown from her arms. As panic ensued, the 15th Hussars and another yeomanry regiment were ordered to disperse the crowd, causing more bloodshed as protesters were trampled and crushed.

Christopher Day, the head of modern domestic records at the National Archives, said: “All of the accounts we have are quite partisan and also it was a very confusing situation. What’s interesting about the stuff that we have is that we have accounts from officials written at the time before this becomes a political issue. It’s their initial reflection.”

He described the note as “one of the first written accounts in the world of the Peterloo massacre”.

Public re-enactment to mark 200th anniversary of Peterloo massacre Read more

The archived documents also include an account of the events published days later in the Manchester Observer, which helped organise the protest. Seeking to combat loyalist propaganda portraying the authorities’ actions as necessary, the radical-supporting liberal paper stressed that the “tragedy” was unprovoked by lawful, peaceful demonstrators. “All the blood which has been spilled,” the paper reported on 21 August 1819, “has been most wantonly and unnecessarily spilled.”

The newspaper was later closed but Peterloo inspired the eventual launch in 1821 of the Manchester Guardian – which later became the Guardian – by the local businessman John Edward Taylor who witnessed the massacre.

Meanwhile, a letter sent months before the massacre in March 1819 from Henry Hobhouse, the then permanent undersecretary of the Home Office, to a Lancashire magistrate gives an insight into the thinking in Whitehall at the time.

Replying to a letter from magistrates detailing disaffection in Lancashire, Hobhouse writes that he and home secretary, Lord Sidmouth, fear that “your country will not be tranquillized, until Blood shall have been shed either by the Law or the sword”.

The archive collection includes a hand-coloured print of Peterloo owned by John Jenkins, a former Royal Marine, who toured the country showcasing pictures. The print was seized in Devon from Jenkins three months after the massacre, when authorities learned he had been showing the engraving and informing viewers “that the slaughter at Manchester was committed by the cavalry”. Jenkins was jailed for five weeks.

Day said: “[The archive documents] show the very real fear and antagonism and animosity on the part of the authorities and reformers, particularly in the northern parts of Britain, which in many places was misplaced and led to this catastrophe.”

He added: “What the other collection shows is just how immediately this is mobilised politically and utilised to make political arguments … the reason why we are still talking about it 200 years later is because [of] the way it was immediately mobilised by both sides.”

• This article was amended on 14 August 2019 because an earlier version was missing the word “past” in the following quote: “The cavalry are now securing the streets in all directions, ½ past 2 o’clock, yours H. Allen”.