Chancellor Angela Merkel - reunited Germany's first leader to grow up in the communist east - started the day with President Horst Koehler and other leaders at a prayer service at a former East Berlin church that was a rallying point for opposition activists in 1989.
"We remember the tears of joy, the faces of delight, the liberation," Lutheran Bishop Wolfgang Huber told the congregation at the Gethsemane Church.
East Germany's fortified border crumbled on the evening of Nov. 9, 1989, after 28 years holding in the country's citizens - a pivotal moment in the collapse of communism in Europe that followed a confused announcement by a senior official.
At the end of a plodding news conference, Politburo spokesman Guenter Schabowski offhandedly said East Germany was lifting restrictions on travel across its border with West Germany.
Pressed on when the regulation would take effect, he looked down at his notes and stammered: "As far as I know, this enters into force ... this is immediately, without delay."
Schabowski has said he didn't know that the change wasn't supposed to be announced until the following morning.
East Berliners streamed toward border crossings. Facing huge crowds and lacking instructions, border guards opened the gates - and the wall was on its way into history.
Merkel said she was among the East Germans who, hearing Schabowski's words, thought "something might happen on the evening of Nov. 9." Like many others, she made her way across.
"We were speechless and happy," the 55-year-old recalled in an interview with ARD television.
Music from Bon Jovi and Beethoven was to recall the joy of the border's opening, which led to German reunification less than a year later and the swift demolition of most of the wall - which snaked around West Berlin, a capitalist enclave deep inside East Germany, for 96 miles (155 kilometers).
Memorials also were planned to the 136 people killed trying to cross the border. Candles were lit and 1,000 towering plastic foam dominoes placed along the wall's route to be tipped over.
Also expected in Berlin for the ceremonies were the leaders of all 27 European Union countries and Russian President Dmitri Medvedev.
By the Brandenburg Gate, the symbol of Germany's division and then of its reunification, which for nearly three decades stood just behind the wall in no man's land, Dieter Mohnka, 74, and his wife Helga, 71, originally from East Berlin, shared a bowl of French fries, and recalled the night the wall was opened.
"We were shocked when we heard that announced, simply astounded," said Helga. "The next morning we went straight to visit my aunt in the West."
Dieter, who taught high school at the time, said he had long been fascinated with West Germany.
"I was born in East Germany, I went to school in East Germany. I was supposed to teach the kids about the wonderfulness of the East, when I was secretly watching TV from the West," he said.
Later Monday, music and fireworks will hark back to the night of Nov. 9, 1989, when people danced atop the Berlin Wall in front of the gate. On that cold night, years of separation and anxiety melted into the unbelievable reality of freedom.
East Germans came in droves, many driving their sputtering Trabant cars. Hundreds of thousands crossed over the following days, as West Berlin stores stayed open late and banks gave out 100 western marks in "welcome money," then worth about $50, to each East German visitor.
By Nov. 12, more than 3 million of East Germany's 16.6 million people had visited the west, while tourists chiseled off chunks of the war to keep as souvenirs.
"That was the day of all days," said Walter Momper, the mayor of West Berlin at the time, who attended a candle lightening ceremony at the wall memorial at Bernauer Strasse.
"I'm happy again every time I remember that we all got to this unification in such a peaceful way," he said.
Hundreds of visitors from around the world lit candles and put them in sand boxes in front of the Bernauer Strasse wall memorial, next to one of the few remaining stretches of the Berlin Wall.
Earlier, a brass orchestra played and many school children placed red and yellow roses at the memorial.
Annika Fischer, a school teacher, showed her class of 10-year-old students a mark on the pavement, where the wall once stood.
"This is where the wall was, and we are now walking from the west into the east," she said, crossing the brick line. "I could not do that 20 years ago."