The fall of the Berlin Wall has shown the world that dreams can come true and "nothing has to stay as it is", German Chancellor Angela Merkel has said.
Speaking 25 years after the event, Mrs Merkel said the message for those in countries where rights were threatened was that things could get better.
Earlier she attended a service for the former East German regime's victims.
The Berlin Wall was built in 1961 to stop people fleeing from communist East Germany to the West.
Its fall in 1989 became a powerful symbol of the end of the Cold War.
Later in the day, some 7,000 white balloons – perched on 3.6-metre poles to match the height of the wall and stretching for 15km (nine miles) – will be released to symbolise its disappearance.
Before that there will be a concert and a "citizen's party" at the Brandenburg Gate.
'We can change things'
The day's events began with a brass band playing, evoking the trumpets which brought down the walls of the biblical city of Jericho.
Chancellor Merkel, who grew up in East Germany, and other officials laid roses in one of the remaining sections of the wall.
Speaking at the opening of a new information centre about the Wall, Mrs Merkel said it was easy to forget what had happened and it was important to remember it.
"We can change things for the better," she said. "This is the message for… Ukraine, Iraq and other places where human rights are threatened.
"The fall of the Wall showed us that dreams can come true. Nothing has to stay as it is."
At the scene: Jenny Hill, BBC News
It's a bitterly cold grey day in Berlin. This morning hundreds of people stood shivering next to one of the few remaining sections of the wall. From a big screen above them Angela Merkel could be seen at a commemorative church service. Later the crowds here will celebrate. But for now the mood is sombre. This wall, after all, dominated Berlin, part of a border which split a country in half. And scores of people died trying to cross it to escape communist East Germany.
One man had tears in his eyes as he told me he was born in East Berlin. People must never forget what the wall represented, he said.
As Angela Merkel left church the crowds surged towards her. Speaking to some of them the chancellor said it was important to remember all those who suffered because of the wall, not just in Germany but across Eastern Europe
Freedom is a concept much valued by Angela Merkel. No wonder, perhaps, given that the German chancellor grew up in the East. She was in her 30s – working as a physicist – when the wall came down.
Recently Ms Merkel has revealed more details about her movements on the day that the Wall opened.
She told German TV on Saturday that she joined crowds heading towards West Berlin after a visit to the sauna, describing "an incredible feeling of happiness".
The chancellor will be joined later at the Brandenburg Gate by former Polish trade union leader and president Lech Walesa and Mikhail Gorbachev, the last Soviet leader.
The wall stretched for 155km (96 miles) through Berlin but today only about three kilometres of it still stands.
At least 138 people died trying to flee to West Berlin.
Within a year of the wall's collapse, Germany – divided after its defeat in World War Two – was reunited.
More than a million visitors have descended on Berlin for the weekend of festivities.
Striking a more sombre note, Mr Gorbachev, 83, warned on Saturday that the world was on the brink of a new Cold War.
Tensions between the West and Russia have been raised by the crisis in Ukraine, which was part of the Soviet Union.
"Bloodshed in Europe and the Middle East against the backdrop of a breakdown in dialogue between the major powers is of enormous concern," he said.
"The world is on the brink of a new Cold War. Some are even saying that it's already begun."
Mr Gorbachev, as leader of the USSR in the late 1980s, is credited with rapprochement with the West and creating a more liberal atmosphere which led to the collapse of communist regimes in Eastern Europe in 1989.