More than 400 people were arrested as violence broke out after thousands of anti-G20 protesters marched through downtown Toronto on Saturday, prompting police to use tear gas in the city for the first time. When the demonstration started just after 1 p.m. ET at the legislature, the mood was upbeat and peaceful. Protesters danced, clapped and chanted while carrying signs, flags and umbrellas as they first moved south down one of the city’s main thoroughfares, University Avenue.But the mood turned ugly as members of splinter groups broke away in attempts to move toward the summit site near Lake Ontario. They used a tactic known as the Black Bloc, in which protesters wearing black hoodies, masks, balaclavas and skateboard helmets engage in violent behavior.