If you are looking for an interesting introduction to Shelley's radical and intensely political poem, the Masque of Anarchy, look no further. This article is produced by the British Library as part of their "Discovering Literature: Romantics and Victorians" series. Professor Mullan is Professor of English at University College London and is a specialist in 18th-century literature.  In 2012 he published "What Matters in Jane Austen?"  I am happy link to this wonderful series because virtually all of the articles, like this one, are written in an approachable, accessible style. 

From the articles Introduction: 

Percy Bysshe Shelley was living in Italy when news reached him of the Peterloo Massacre. On 16 August 1819 a crowd of well over 50,000 had gathered at St. Peter’s Fields outside Manchester to support parliamentary reform. The radical orator Henry Hunt was to speak in favour of widening the franchise and reforming Britain’s notoriously corrupt system of political representation, with its ‘pocket’ and ‘rotten’ boroughs. Magistrates ordered the Manchester Yeomanry (recruited from amongst the local middle classes) to disperse the demonstration. The cavalry charged the crowd, sabres drawn. At least 15 demonstrators, including a woman and a child, were killed, and many more wounded.