To cut a long story short :
In 1819, many people gather in St Peters Field to discuss how they wanted changed to workers rights and the availity to vote for MP's
Unfortunately the local militia (part-time soldiers) got a trifle carried away in keeping order (probably most were drunk anyway), and started hacking people left, right and centre.
11 people died, 400/500 were wounded (for more details see
http://en.wikipedia.org/wiki/Peterloo)
I do recommend the Horrible History series of books for learning about History (mostly English, but some books detail Aztecs, Egyptian, Greek, and, for Welsh people, Celts).
As some wise person said (could have been me) : You only know where your going, when you've seen where you've been.