São Paulo holds a yearly event called Virada Cultural, which this year was held on the 15th and 16th of May, 2010. The event is organized by the São Paulo City Hall, and presented in different squares all over São Paulo city.
Pity I only caught the news this week, but it was quite a success. For next year´s dates, check the Virada Cultural website. The people I know who went this year only complained of the difficulty in getting from one square to the other in record time, to catch the various shows.