Crowds surged to a series of concerts, discussions, parades and beer along the Unter den Linden avenue leading to the landmark Brandenburg Gate which for years stood out of reach behind the Berlin Wall and was a symbol of the nation's division.
That wall was torn down 20 years ago this November, along with the Iron Curtain that divided Europe.
Daniel Barenboim, musical director of the Staatsoper Unter den Linden, and singer Udo Jürgens were among several musicians to perform throughout the day.
Police also expected around 2,000 leftists to attend a counter “antinationalist” parade.
Germany's constitution or “basic law” established the post-World War II federal republic. Originally known abroad as West Germany, it expanded to incorporate the formerly communist east when the country was reunited in 1990.
Specifically, the events celebrate the 1949 creation of a democratic nation that still bore the shame of the Nazis' horrors and was struggling to rebuild after total defeat and destruction in 1945.
Other events to mark this milestone are planned across Germany in 2009, which also sees the 20th anniversary of the fall of the Berlin Wall before the unification of West and East Germany in 1990 - all of this in an election year.
"The founding fathers of the constitution created a solid order so that free citizens could create a life for themselves in a just society," German President Horst Köhler said during a service in the Berlin cathedral on Saturday morning.
Chancellor Angela Merkel said Germany's constitution was founded on the principle of upholding human dignity.
“We are celebrating freedom, unity, democracy. The key to this great happiness lies in one significant sentence: ‘Human dignity shall be inviolable,'” Chancellor Angela Merkel wrote in an article published on Saturday in the Berliner Zeitung.
On Friday, Köhler said Germany's constitution is a “beacon of democracy.”
“In the past 60 years we have turned into an open and cosmopolitan society,” the president said during a speech at a Berlin concert hall.
Köhler called on Germany to continue the process of re-unification. “Unity is like democracy: it is never finished,” he said. “It is has to be lived, tested, constantly re-examined in our everyday lives.”