In March 1819, Joseph Johnson, John Knight and James Wroe formed the Manchester Patriotic Union Society. All the leading radicals in Manchester joined the organisation. Johnson was appointed secretary and Wroe became treasurer. The main objective of this new organisation was to obtain parliamentary reform and during the summer of 1819 it was decided to invite Major Cartwright, Henry Orator Hunt and Richard Carlile to speak at a public meeting in Manchester. The men were told that this was to be "a meeting of the county of Lancashire, than of Manchester alone. I think by good management the largest assembly may be procured that was ever seen in this country." Cartwright was unable to attend but Hunt and Carlile agreed and the meeting was arranged to take place at St. Peter's Field on 16th August.
The local magistrates were concerned that such a substantial gathering of reformers might end in a riot. The magistrates therefore decided to arrange for a large number of soldiers to be in Manchester on the day of the meeting. This included four squadrons of cavalry of the 15th Hussars (600 men), several hundred infantrymen, the Cheshire Yeomanry Cavalry (400 men), a detachment of the Royal Horse Artillery and two six-pounder guns and the Manchester and Salford Yeomanry (120 men) and all Manchester's special constables (400 men).
At about 11.00 a.m. on 16th August, 1819 William Hulton, the chairman, and nine other magistrates met at Mr. Buxton's house in Mount Street that overlooked St. Peter's Field. Although there was no trouble the magistrates became concerned by the growing size of the crowd. Estimations concerning the size of the crowd vary but Hulton came to the conclusion that there were at least 50,000 people in St. Peter's Field at midday. Hulton therefore took the decision to send Edward Clayton, the Boroughreeve and the special constables to clear a path through the crowd. The 400 special constables were therefore ordered to form two continuous lines between the hustings where the speeches were to take place, and Mr. Buxton's house where the magistrates were staying.
The main speakers at the meeting arrived at 1.20 p.m. This included Henry 'Orator' Hunt, Richard Carlile, John Knight, Joseph Johnson and Mary Fildes. Several of the newspaper reporters, including John Tyas of The Times, Edward Baines of the Leeds Mercury, John Smith of the Liverpool Mercury and John Saxton of the Manchester Observer, joined the speakers on the hustings.
At 1.30 p.m. the magistrates came to the conclusion that "the town was in great danger". William Hulton therefore decided to instruct Joseph Nadin, Deputy Constable of Manchester, to arrest Henry Hunt and the other leaders of the demonstration. Nadin replied that this could not be done without the help of the military. Hulton then wrote two letters and sent them to Lieutenant Colonel L'Estrange, the commander of the military forces in Manchester and Major Thomas Trafford, the commander of the Manchester & Salford Yeomanry.
Major Trafford, who was positioned only a few yards away at Pickford's Yard, was the first to receive the order to arrest the men. Major Trafford chose Captain Hugh Birley, his second-in-command, to carry out the order. Local eyewitnesses claimed that most of the sixty men who Birley led into St. Peter's Field were drunk. Birley later insisted that the troop's erratic behaviour was caused by the horses being afraid of the crowd.
The Manchester & Salford Yeomanry entered St. Peter's Field along the path cleared by the special constables. As the yeomanry moved closer to the hustings, members of the crowd began to link arms to stop them arresting Henry Hunt and the other leaders. Others attempted to close the pathway that had been created by the special constables. Some of the yeomanry now began to use their sabres to cut their way through the crowd.
When Captain Hugh Birley and his men reached the hustings they arrested Henry Hunt, John Knight, Joseph Johnson, George Swift, John Saxton, John Tyas, John Moorhouse and Robert Wild. As well as the speakers and the organisers of the meeting, Birley also arrested the newspaper reporters on the hustings.
Lieutenant Colonel L'Estrange reported to William Hulton at 1.50 p.m. When he asked Hulton what was happening he replied: "Good God, Sir, don't you see they are attacking the Yeomanry? Disperse them." L'Estrange now ordered Lieutenant Jolliffe and the 15th Hussars to rescue the Manchester & Salford Yeomanry. By 2.00 p.m. the soldiers had cleared most of the crowd from St. Peter's Field. In the process, 18 people were killed and about 500, including 100 women, were wounded.
Richard Carlile managed to avoid being arrested and after being hidden by local radicals, he took the first mail coach to London. The following day placards for Sherwin's Political Register began appearing in London with the words: 'Horrid Massacres at Manchester'. A full report of the meeting appeared in the next edition of the newspaper. The authorities responded by raiding Carlile's shop in Fleet Street and confiscating his complete stock of newspapers and pamphlets. Carlile was later imprisoned for publishing this story.
James Wroe was at the meeting and he described the attack on the crowd in the next edition of the Manchester Observer. Wroe is believed to be the first person to describe the incident as the Peterloo Massacre. Wroe also produced a series of pamphlets entitled The Peterloo Massacre: A Faithful Narrative of the Events. The pamphlets, which appeared for fourteen consecutive weeks from 28th August, price twopence, had a large circulation, and played an important role in the propaganda war against the authorities. Wroe, like Carlile, was later sent to prison for writing these accounts of the Peterloo Massacre.
Moderate reformers in Manchester were appalled by the decisions of the magistrates and the behaviour of the soldiers. Several of them wrote accounts of what they had witnessed. Archibald Prentice sent his report to several London newspapers. When John Edward Taylor discovered that John Tyas of The Times, the only reporter from a national newspaper at the meeting, had been arrested and imprisoned, he feared that this was an attempt by the government to suppress news of the event. Taylor therefore sent his report to Thomas Barnes, the editor of The Times. The article that was highly critical of the magistrates and the yeomanry was published two days later.
After the Peterloo Massacre Viscount Sidmouth, the Home Secretary, sent a letter of congratulations to the Manchester magistrates for the action they had taken. Parliament also passed the Six Acts in an attempt to make sure reform meetings like the one at St. Peter's Field did not happened again.
The trial of the organisers of the St. Peter's Field meeting took place in York between 16th and 27th March, 1820. The men were charged with "assembling with unlawful banners at an unlawful meeting for the purpose of exciting discontent". Henry Hunt was found guilty and was sent to Ilchester Gaol for two years and six months. Joseph Johnson, Samuel Bamford and Joseph Healey were each sentenced to one year in Lincoln Prison. John Saxton, John Moorhouse, George Swift and Robert Wild were acquitted.