The meeting had been organized by the Manchester Patriotic Union Society, a political group that agitated for the repeal of the corn laws and parliamentary reform. They had invited a number of speakers, including Richard Carlile, John Cartwright and Henry Hunt, to a public meeting.
Local magistrates, under chairman William Hulton, were concerned that the meeting would end in a riot or, worse, a rebellion. They arranged for substantial number of regular soldiers to be on hand. The troops included 600 men of the 15th Hussars; several hundred infantrymen; a Royal Horse Artillery unit with two six-pounder guns; 400 men of the Cheshire Yeomanry Cavalry; 400 special constables; and 120 cavalry of the Manchester and Salford Yeomanry, relatively inexperienced militia recruited from among shopkeepers and tradesmen.
A considerable crowd from all around the county of Lancashire had gathered for the meeting; contemporary estimates varied from 30,000 up to 150,000; modern estimates are around 60,000 or 80,000. People expected a peaceful meeting and many were wearing their Sunday clothes. Some carried banners with texts like "No Corn Laws", "Annual Parliaments", "Universal Suffrage" and "Vote By Ballot." The main speakers did not arrive until after 1:00 pm, and Hunt was invited to speak first at 1:20 pm.
At around 1:30 pm the magistrates observing the meeting decided to stop the meeting. When the reading of the Riot Act did not help, they gave orders to Captain Joseph Nadin, Deputy Constable of Manchester, to arrest the leaders. Nadin requested military aid and magistrates sent for the Manchester and Salford Yeomanry.
Sixty Yeomanry cavalrymen, possibly drunk, entered the field under their leader Captain Hugh Birley, brandishing their cavalry sabres and charging towards the cart that served as the speakers' stand. When some demonstrators tried to stop them by linking their hands, they begun to attack them with their sabres. When the cavalry reached the cart, they arrested Hunt, Joseph Johnson and number of others, including some newspapermen.
The Yeomanry then begun to strike down the flags and banners of the crowd with their sabres. Outside the field William Hulton perceived the crowd's actions as an assault and ordered Lieutenant Colonel Guy L'Estrange of the Hussars into the field at 1:50 pm, ostensibly to save the Yeomanry. Within ten minutes the Hussars had cleared the field and also pacified the yeomanry.
Eleven people were killed, including a woman, a child another woman was carrying, and a peace officer. About 400 were injured, 100 of them women, many of whom were trampled by horses. One man had his nose severed, and others were bleeding from numerous sabre cuts. Even some local masters, employers and owners were horrified of the carnage.
The events immediately found their way into the press. James Wroe of the Manchester Observer coined the phrase "Peterloo Massacre" to describe the event (in reference to Waterloo). Sympathetic Richard Carlile avoided arrest and published the story in his Sherwin's Political Register. Both Wroe and Carlile were later imprisoned for publishing the story.
The government supported the action of the army and magistrates. The Home Secretary, Lord Sidmouth, even congratulated them. By the end of the year, the government had introduced legislation, later known as the Six Acts, to suppress radical meetings and publications. The widespread public anger at the massacre swelled the support of the reform movement from which the Chartists would eventually emerge.
The maximum penalty under the Riot Act would have been the death sentence. However, Henry Hunt was sentenced to 30 months in Ilchester Jail. Others received a year each or were acquitted. Hunt was later released on bail.
No public inquiry was allowed until 1820. The first Parliamentary Reform Act began in 1832.