Simplify the following text for a Grade 10 native English-speaking person.
Original: The Peterloo Massacre ( or Battle of Peterloo ) occurred at St Peter 's Field , Manchester , England , on 16 August 1819 , when cavalry charged into a crowd of 60,000 â `` 80,000 gathered at a meeting to demand the reform of parliamentary representation .
Simplified: The Peterloo Massacre ( or Battle of Peterloo ) happened at St Peter 's Field , Manchester , England , on 16 August 1819 . It was when cavalry charged into a crowd of 60,000 to 80,000 gathered at a meeting to demand the reform of parliamentary representation .