Mrs Clinton also spoke, before introducing a surprise video address from Washington by Mr Obama.
"There could be no clearer rebuke of tyranny. There could be no stronger affirmation of freedom," he said of the wall's tearing down.
Mr Medvedev said the wall's collapse had helped Russia and Germany end their World War II enmity.
He said he hoped everyone had rejected the dividing lines represented by the wall.
In his speech, Mr Brown told Berliners: "You dared to dream in the darkness. You knew that while force has the temporary power to dominate, it can never ultimately dictate."
After the leaders spoke, a chain of 1,000 giant foam dominoes - painted with messages of freedom by young people - was toppled along where the wall once stood.
The symbolic act was to reflect how the then Communist governments of Eastern Europe fell one after another.
The first wave of dominoes were knocked over by former Polish President Lech Walesa and ex-Hungarian Prime Minister Miklos Nemeth.
Mr Nemeth's decision to open his country's borders first allowed East Germans to flee to the West.
The festivities were capped with fireworks and a concert featuring music from Berlin's State Opera and the American rock band Bon Jovi.
Earlier in the day, Ms Merkel retraced her steps on the night the wall fell by crossing the first checkpoint to open 20 years ago.
The German chancellor was joined by ex-Soviet leader Mikhail Gorbachev and Mr Walesa, the former Polish trade union leader and later president, whose movement was the first to challenge Communist rule successfully in Eastern Europe.
There were cheers from hundreds of onlookers as Ms Merkel praised Mr Gorbachev for helping to make change possible.
Earlier, Ms Merkel said in a TV interview that German unity was still incomplete, as the East lagged in economic growth.
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