With the cry of "O'zapft is" ("The keg is tapped"), the amber fluid began to flow at noon after Munich's mayor, Dieter Reiter, with due pomp and ceremony, took a mallet and in four swings breached a 200-litre (53-gallon) barrel.
The first litre-size mug, the "Mass", went to the head of the Bavaria regional government, Horst Seehofer.
Fourteen giant tents set up in the centre of Munich filled up quickly with people eager to imbibe the slightly stronger than usual beer after watching a rain-dampened parade of the brewers through the city's streets.
"Oktoberfest is very famous, everyone in the world knows about it. We wanted to see it and now I can tell my friends I've been too," said Moran Chen, 26, originally from Hong Kong but now living in Berlin.
At the festival site on the Wiesn fairgrounds thousands of serving staff carry millions of "Mass" glasses of beer from one of six historic Munich breweries to punters seated at long tables.
Last year more than six million visitors drank 6.7 million litre-sized mugs of beer — more than twice the volume of an Olympic swimming pool — during the whole festival run.
The price of the beer always touches off a debate and in some tents this year it will creep over the 10-euro mark for the first time — €10.10 ($13), according to officials.
It all helps wash down the salty giant soft pretzels, dumplings, pork and grilled sausages that provide the customary hearty accompaniment.
For Johanna Kriessl, a 53-year-old from Frankfurt now living in Munich, its magic is the "super, nice atmosphere" and "the tradition of the tracht", referring to the traditional Bavarian costume.
Oktoberfest customs call for the wearing of classic southern Bavarian dress — a dirndl, or low-cut blouse with a laced-up bodice and aproned full skirt for women, and the lederhosen leather shorts with embroidered braces for men.
Foreign tourists, led by Italians and Americans, flock to Munich to enjoy the merriment, but the festival remains largely a Bavarian event with more than 70 percent of visitors hailing from the southern state.
"For us it is wonderful for the whole Bavarian nation to gather together in such a small space," said Munich resident Barbara Huber, 50.
And she was insistent that the festival should not be seen as quintessentially German. "No, it's Bavarian, it's Munich."
It does, however, draw plenty of German celebrities and stars of the TV screen and soccer field, who are often photographed, glass of beer in hand.
The Oktoberfest, originally held in October as the name suggests but brought forward to September to take advantage of warmer weather, began in 1810 to mark the marriage of the prince of Bavaria to Princess Therese of Saxony-Hildburghausen.
This year is the 181st edition of the legendary festival which was cancelled during two cholera outbreaks, both world wars, Napoleon's invasion of Bavaria and the hyperinflation of the 1920s.
It has since been exported around the world and versions of the festival can be found as far afield as China, Brazil, Canada, the United States, Russia and Australia.
This year's event runs to October 5.