A public reform meeting was called, to be held on 8th January 1819 in St Peters Field (now St Peters Street), as there was no building thought big enough to hold the anticipated crowd. Henry Hunt, a national reform leader, and noted orator who had spoken elsewhere that year, was to address the crowd. Estimates put the crowd at variously 30,000 and 150,000 people - in any case, we can be certain that there were more people present than Manchester had ever seen in one place before. Disturbed by such large crowds, magistrates called in local militia to stand ready. Some 1500 troops assembled, comprising the 15th Hussars (professional soldiers) and soldiers of the Manchester and Cheshire Yeomen Cavalry (a largely volunteer force), commanded by Lieutenant-Colonel Guy L'Estrange. Magistrates, fearing insurrection and riots, ordered Hunt and other leaders to be arrested before they could speak, though the meeting had been thus-far peaceful and orderly.

Inadvertently a mounted solder brushed and knocked down a mother, killing the child she was carrying, and panic ensued. Magistrates, and the troop commander, misread what appeared to be a riotous outbreak, and ordered Yeomanry, who were standing ready just off Portland Street, to go in to break up the affray. The armed cavalry, sabres drawn, charged the crowd, cutting people down indiscriminately. Men, women and children were hacked down or trampled by horses or people in flight. After ten minutes of havoc and slaughter, the field was deserted except for the broken hustings platform, bodies of the dead, wounded and dying. A soldier of the Yeomanry company, who had fought at Waterloo in 1815, likened the carnage to that battlefield, and the term "Peterloo" took hold, and survives as an historic event even today. After news of the massacre spread across Britain, local authorities clamped down on all public meetings, in breach of all laws to the contrary, and took severe measures to ensure public order. It had been arguably the most important day in Manchester's political history. Rumours spread that the attack had been planned and may possibly have been ordered, well in advance of the event, by the government in London. Fears of the recent French Revolution permeated British political awareness to such an extent, that the authorities were paranoiac in case ordinary people followed the French example - local authorities had been ordered to stamp out any risk at source and summarily. Another viewpoint blames bystanders throwing stones provocatively at the troops. Debates on the causes of the Massacre continue. Meanwhile, Hunt and the other Peterloo leaders had been incarcerated in Lancaster Castle prison. Thousands of supporters lined the streets when, after being released on bail, Hunt made the return trip to Manchester. Hunt had always insisted on peaceful and legal means to achieve political change, despite being urged to armed protest by many of his supporters.