Valparaiso is probably the most popular and wild urban mountain bike race. It takes place in the Chilean city which gives the name to the race. It starts from the upper town and goes through stairs, tiny streets until sea level. Full of crowd sometimes quite close to the track, racers have to be very talented not to crash.
If you want to feel how it is to be on the bike driving down, just enjoy the video from this year winner run. Tomáš Slavik