Concerts and exhibitions are being staged in the city and Chancellor Angela Merkel will later attend a huge open-air party at the Brandenburg Gate.
White balloons marking a stretch of the wall will be released to symbolise its disappearance.
The Berlin Wall was built in 1961 to stop people fleeing from Communist East Germany to the West.
Its fall in 1989 became a powerful symbol of the end of the Cold War.
Chancellor Merkel will be joined for the festivities by former Polish trade union leader Lech Walesa and Mikhail Gorbachev, the last Soviet leader.
The wall stretched for 155km (96 miles) through Berlin but today only about three kilometres of it still stands, BBC reports.
Within a year of its collapse, Germany - divided after its defeat in World War Two - was reunited.
More than a million visitors have descended on Berlin for the weekend of festivities that will culminate later on Sunday at the Brandenburg Gate.
The monument itself was inaccessible during the partition of Germany and is seen as a symbol of the country's reunification.