Complex sentence: Toronto hosted the G-20 summit on June 26--27,2010, but it was not without protests, which needed one of the most expensive temporary security operations in Canada's history and also had the largest mass arrests in Canadian history.
Split into simpler sentences: Toronto hosted the G-20 summit on June 26--27,2010, but it was not without protests. The protests were met with one the most expensive temporary security operations seen in Canada and resulted in the largest mass arrests in Canadian history.