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People are seen walking around large plastic-foam dominoes, prior to commemorations of the 20th anniversary of the fall of the Berlin Wall on Monday. ((Michael Sohn/Associated Press))

Former Polish leader Lech Walesa tipped the first of 1,000 plastic foam dominoes set up where the Berlin Wall once stood as part of celebrations Monday marking the 20th anniversary of its fall.

The 2.3-metre-high blocks, painted by schoolchildren and stretching for 1.5 kilometres near the Brandenburg Gate and the German parliament, fell one by one, with pre-planned stops for speeches and memorials.

The dominoes were conceived as a metaphor for the collapse of communism in the countries of eastern Europe after the Wall crumbled.

Earlier, German Chancellor Angela Merkel retraced her steps from two decades ago, crossing the Bornholmer Strasse bridge from East Berlin to West Berlin.

Merkel, the first leader of reunited Germany who grew up in communist East Germany, took the ceremonial walk with former Soviet Union leader Mikhail Gorbachev and Walesa, two key figures in the end of the communist system.

She called the fall of the Wall an "epic" moment in history.

"For me, it was one of the happiest moments of my life," Merkel said.

The leaders of all 27 European Union countries as well as Russian President Dmitry Medvedev attended Monday's anniversary ceremonies in Berlin.

"Nov. 9, 1989 will always be remembered and cherished in the United States," U.S. President Barack Obama said in a video message broadcast at the event. "Like so many Americans, I'll never forget the images of people tearing down the Wall. There could be no clearer rebuke of tyranny, there could be no stronger affirmation of freedom."

Merkel was one of thousands who crossed the boundary on the night of Nov. 9, 1989.

"Before the joy of freedom came, many people suffered," she recalled Monday.

Walesa's Polish pro-democracy movement, Solidarity, played a key role in undermining communism in eastern Europe, and Merkel praised Gorbachev for pushing reforms in the Soviet Union and allowing the communist bloc to disintegrate peacefully.

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Former Soviet leader Mikhail Gorbachev, German Chancellor Angela Merkel and former Polish president Lech Walesa stand in front of a picture showing Germans celebrating the fall of the Berlin Wall 20 years ago. ((Gero Breloer/Associated Press))

"We always knew that something had to happen there so that more could change here," she said. "You made this possible — you courageously let things happen, and that was much more than we could expect," she told Gorbachev as several hundred people lining the bridge cheered "Gorby! Gorby!"

The bridge crossing was one of several events marking the anniversary of the fall of the Wall. The opening of the 48-year-old concrete barrier between East and West Germany in the fall of 1989 was a key event in the collapse of communism and led to the reunification of Germany.

Concrete symbol of Cold War

The Wall started in August 1961 as a barbed-wire fence erected by East Germany overnight to isolate West Berlin from communist-controlled East Berlin and the rest of East Germany. Much of it was eventually replaced by a concrete structure, and West Berlin was encircled by the 155-kilometre Wall, of which 43 kilometres separated the east and west sides of the city.

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A West Berliner swings a sledgehammer, trying to destroy the Berlin Wall near Potsdamer Platz on Nov. 12, 1989. Berliners from both east and west began pulling down the infamous wall three days earlier. ((John Gabb/Associated Press))

It fell on Nov. 9, 1989, after Politburo spokesman Guenter Schabowski announced at the end of a news conference that East Germans would be allowed to travel across the border to West Germany. Asked when that would happen, Schabowski mistakenly said it would take effect immediately, though the change was not supposed to be enacted until the following day.

East Berliners responded, flooding toward border crossings, and border guards, overwhelmed and lacking official orders specifying what was to be done, opened the gates.

Sabine Sparwasser, now the German consul general in Toronto, was visiting West Berlin on that night and recalls hearing the noise in the streets from her hotel room.

"I looked out, and I saw the cars, and I thought, these aren’t supposed to be here, these are East German cars," she told CBC's Suhana Meharchand.

"And I went out and walked the street, and there were people out in nightgowns, their curlers, everybody brought out what they had for drinks in the house, and we wandered through the night and cried and hugged. It was beautiful."

By Nov. 12, more than three million of East Germany's 16.6 million people had visited the West while tourists chiselled off chunks of the Wall to keep as souvenirs.

With files from The Associated Press