Here's a graphic video clip of the Shi'a ritual of Ashura from the Washington Post's Jack Fairweather, who is traveling around Central Asia and the Middle East in search of answers to the question of what does Islam mean to young people. The clip is rather amateurish and I don't know if it was because videotaping the ritualistic flagellation is prohibited.
Nevertheless, it's an unusual glimpse in a ritual which has its counterpart in Catholicism. It takes place amongst the Shia communities of the Middle East and beyond, and Ashura is on the 10th day of Muharram in the Islamic calendar.
It is commemorated by the Shi‘a as a day of mourning for the martyrdom of Hussein ibn Ali, the grandson of the Islamic prophet Muhammad at the Battle of Karbala on 10 Muharram in the year 61 After Hijra ( corresponding to December 10, 680 AD).
Sunni Muslims do not self flagellate (in fact they consider it as barbaric), but believe that Moses fasted on that day to express gratitude to God for liberation of Israelites from Egypt. According to Sunni Muslim tradition, Muhammad fasted on this day and asked other people to fast as well.
Washington Post's Islam's Advance article.