Global Marijuana March has been going on since 1999.

2005 reports, photos, and links from around 180 cities worldwide:
http://members.fortunecity.com/multi19/mmm2005map.htm

Million Marijuana March. World Cannabis Day. Cannabis Liberation Day. Global Marijuana March. Worldwide since 1999. The first Saturday in May, or that weekend, or thereabouts. Rallies, marches, concerts, events, meetings, parties, raves, info-tables, stands, booths, etc..

2006 city list:
http://www.globalmarijuanamarch.org