Here's some BBC links (http://www.bbc.co.uk/news/uk-22644057)
(http://www.bbc.co.uk/news/uk-22664835)
Some of the arrests were before two competing groups held demonstrations in the same town. The English Defence League (hateful moronic racist thugs) and Anti-Facist something or other (a rag-tag bag of anarchists, tin pot trots, students, and anti-facists) were both holding demonstrations.
Police are perhaps a bit antsy because both groups have had problems with violence, and this is exacerbated when they demonstrate at the same time.
US citizens may not like our laws, but we -on the whole- do. It is a weird cultural difference between the UK and US.