Mayor Dieter Reiter opened Munich's two-week Oktoberfest beer festival Saturday, shouting "O'zapft is!" as he tapped the first keg. Persistent rain failed to dampen the spirits of festival-goers as thousands queued up at beer tents for the festival's 181st celebration.

Deutsche Welle reported some tents were so overcrowded, they had to be closed off to new arrivals -- even before the festival officially opened.

The beer is supplied by six breweries, all Bavarian, the reported. The region takes its beer seriously, introducing the Reinheitsgebot or purity law in 1516, considered the world's first consumer protection law. Then in 1539, it was decreed beer could be brewed only from September to April, but that law has gone by the boards.

Oktoberfest beer gets more alcohol than brews at other times of the year, the Local said. It's also lighter, sweeter and less carbonated.

tag-reuters (3) A general view of the festival ground during the opening day of the 181st Oktoberfest in Munich. Photo: Reuters/Lukas Barth

Some 6 million of beer drinkers from around the world are expected in the Bavarian capital for the Oktoberfest, which runs to Oct. 5.

tag-reuters (5) People ride an attraction in front of Munich's famous Frauenkirche church. Photo: Reuters/Lukas Barth

tag-reuters (6) Balloons in the Bavarian colors are seen at a booth. Photo: Reuters/Lukas Barth

tag-reuters (7) People rest against a wall. Photo: Reuters/Lukas Barth

tag-reuters (8) Visitors ride a merry-go-round. Photo: Reuters/Lukas Barth

tag-reuters (9) A gingerbread heart. Photo: Reuters/Lukas Barth

tag-reuters (10) Newly elected mayor of Munich Dieter Reiter (C) speaks while being interviewed during the opening ceremony. Photo: Reuters/Lukas Barth

tag-reuters (11) Visitors enjoy a beer. Photo: Reuters/Lukas Barth