The main celebrations in the city will be at the Brandenburg Gate – the symbol of German reunification in 1990.
Giant dominoes will be toppled to show how Communist governments in Eastern Europe fell one after another in 1989.
Communist East Germany erected the 155km (96-mile) concrete Wall in 1961 to encircle West Berlin.
It was put up to prevent East Germans from fleeing into the capitalist enclave.
More than 100 people are believed to have been killed at the Wall while trying to escape.
The barrier was unexpectedly opened on 9 November 1989, following weeks of pro-democracy protests.
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Berliners celebrate on top of the dismantled Wall. Photo: 12 November 2009
German Chancellor Angela Merkel, who grew up in East Germany, will lead the celebrations on Monday.
Last week, she said the fall of the Wall was "the happiest day in recent German history".
Among those invited are French President Nicolas Sarkozy, British Prime Minister Gordon Brown and US Secretary of State Hillary Clinton.
They also will be joined by:
* Former Polish President Lech Walesa, who led the Solidarity free-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
# Former Soviet leader Mikhail Gorbachev, who let the nations of Eastern Europe go their own way
‘New push’
As part of festivities, some 1,000 giant foam dominoes – painted by young people – will be toppled along the former route of the Wall.
"History is palpable and alive here. The peaceful revolution of the fall of the Wall 20 years ago paved the way to an unprecedented transformation of Berlin," Mayor Klaus Wowereit said.
A big concert at the Brandenburg Gate is due to start later in the day.
The festivities will be capped with fireworks and performances by musicians from all over the world.
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.
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