World leaders are joining thousands of people marking 20 years since the Berlin Wall's fall, an event that paved the way for the end of the Cold War.
German Chancellor Angela Merkel, who grew up in East Germany, is leading the celebrations.
Guests including ex-Soviet leader Mikhail Gorbachev will join her in walking across the first border crossing to open in 1989.
The barrier was unexpectedly opened after weeks of pro-democracy protests.
Communist East Germany erected the 155km (96-mile) concrete wall in 1961 to encircle West Berlin and prevent citizens from fleeing into the capitalist enclave. More than 100 people are believed to have been killed at the Wall while trying to escape.
The main events marking the anniversary in the city will be at the Brandenburg Gate - the symbol of German reunification in 1990.
As part of festivities, some 1,000 giant foam dominoes - painted by young people - will be toppled along the former route of the Wall, to symbolise how communist governments in Eastern Europe fell one after another in 1989.
Among those invited to join Chancellor Merkel in Berlin are French President Nicolas Sarkozy, British Prime Minister Gordon Brown and US Secretary of State Hillary Clinton.
They will be joined by:
former Polish President Lech Walesa, who led the Solidarity trade union against the Communist regime
former Hungarian Prime Minister Miklos Nemeth, whose decision to open his country's borders first allowed East Germans to flee to the West
A big concert is due to start later in the day. The festivities will be capped with fireworks and performances by musicians from across the world.
Meanwhile, Guenter Schabowski - the former East German official whose casual public comments about the proposed ending of restrictions on travel from East to West Berlin are widely thought to have led to the fall of the Wall - has admitted there was a breakdown in communication with his party boss, Egon Krenz.
Mr Schabowski blurted out the plans during a televised press conference - and compounded his error by adding the new rules would come into force "immediately".
He told the BBC he did not regret his so-called blunder because it had led to the divided Germany being reunited peacefully.
On the eve of the celebrations, Mrs Clinton called for a new push to free those still oppressed.
"Our history did not end the night the wall came down," she said.
"To expand freedom to more people, we cannot accept that freedom does not belong to all people."
