Carlile was one of the scheduled main speakers at the reform meeting on 16 August 1819 at St. Peter's Fields in Manchester. Just as Henry Hunt was about to speak, the crowd was attacked by the yeomanry in what became known as the Peterloo massacre. Carlile escaped and was hidden by radical friends before he caught the mail coach to London and published his eyewitness account, giving the first full report of what had happened, in "Sherwin's Weekly Political Register" of 18 August 1819. His placards proclaimed "Horrid Massacres at Manchester".