in which 39 football
to North Americans) fans were crushed to death
inside Heysel Stadium in Brussels
before the 1985 European Cup
Final between Liverpool
(of England) and Juventus
While details are somewhat sketchy, the general consensus is that fans of both teams were involved in a series of fights before the match. A large group of Liverpool supporters charged forward and the Juventus fans backed up and became trapped against a wall. The wall collapsed, killing 39, mostly Italians. The game was played anyway (with Juventus winning 1-0).
As a result of the disaster, which was blamed on English hooliganism, clubs from England were banned from European tournaments for 5 years.
While English hooliganism has somewhat declined since the disaster and its aftermath, there's still often violence involving English fans (such as in the 2000 European Championships, and between English fans and Galatasaray supporters in 2000), and worldwide there have been many deaths at football games due to rowdy fans and poor planning and security.