Germany has kicked off celebrations marking 25 years since the epochal fall of the Berlin Wall, set to culminate in rock stars and freedom icons joining millions at an open-air party.

Chancellor Angela Merkel, who grew up in communist East Germany, is leading three days of commemorations for those killed trying to flee the repressive state, ahead of a giant festival Sunday marking the joyous breach of Europe's Cold War division on November 9, 1989.

"I think you never forget how you felt that day -- at least I will never forget it," Merkel, 60, said in her latest podcast.

"I had to wait 35 years for that feeling of liberty. It changed my life."

The festivities under the banner "Courage for Freedom" recall the peaceful revolution that led communist authorities to finally open the border after 28 years in which Easterners were prisoners of their own government.

Germany would reunite within the year, on October 3, 1990.

The last Soviet leader, Mikhail Gorbachev, whose "perestroika" and "glasnost" reforms helped pave the way for the Wall's fall, on Friday greeted crowds at Berlin's iconic former Checkpoint Charlie border crossing.

US President Barack Obama told the world to remember lessons from the emotional fall of the Berlin Wall.

"I will never forget the scenes of East Berliners courageously taking to the streets, pushing past the guards and tearing down the wall that for so long had separated them from family and friends and the free world," Obama said in a statement.

"As Russia's actions against Ukraine remind us, we have more work to do to fully realize our shared vision of a Europe that is whole, free and at peace," he added.

Relations between Russia and the United States are at their iciest since the end of the Cold War after Moscow annexed Ukraine's Crimean peninsula in March.

'Proud I could contribute'

Gorbachev, 83, who is revered in Berlin for having refrained from a bloody crackdown on protesters in 1989, understated his own role in history, saying: "I am proud I could contribute a little bit to the fact that we live like this today".

But the Nobel Prize winner also warned of new East-West tensions sparked by the Ukraine crisis, saying both sides must "get a grip on the tensions that have emerged recently".

On Saturday, Gorbachev will join former German foreign minister Hans-Dietrich Genscher for a debate about the legacy of 1989, and the resurgence of tensions between Russia and the West.

Berlin has set up an ambitious installation featuring nearly 7,000 white balloons pegged to the ground along a 15-kilometre (nine-mile) stretch of the Wall's former 155-kilometre path.

The glowing orbs, which from above look like a string of pearls, are to be released Sunday and set to float into the night sky, to the stirring strains of Beethoven's "Ode to Joy".

Despite the party mood, old tensions resurfaced earlier Friday when dissident singer Wolf Biermann, who was kicked out of East Germany in 1976, performed his protest song "Ermutigung" (Encouragement) in parliament, in a tribute to those who resisted the regime.

He used the opportunity to take a swipe at the far-left Linke, which has roots in East Germany's ruling party and had criticised Biermann's invitation to appear at the session.

"Your punishment is to have to listen to me here -- enjoy," he told heckling Linke deputies, calling them "the wretched remains of what has fortunately been surmounted".

Freedom icons

Merkel will on Saturday attend a memorial concert at Bertolt Brecht's historic Berliner Ensemble theatre opposite the former "Palace of Tears", where Easterners said goodbye to visitors returning to West Germany.

And on Sunday she will open a major exhibition on Bernauer Strasse, a street divided by the Wall that saw harrowing scenes of families ripped apart overnight when the Wall went up in 1961.

Following a ceremony at the elegant Gendarmenmarkt square, at least two million people are expected to gather at the Brandenburg Gate, the symbol of German unity.

There, Gorbachev and former Polish president and freedom icon Lech Walesa, 71, will join German head of state Joachim Gauck, 74, a former pastor and rights activist in the East, and Hungarian ex-premier Miklos Nemeth, 66.

Entertainment will range from the Berlin State Orchestra to a fireworks display.

British singer-songwriter Peter Gabriel will perform the Wall anthem "Heroes", which David Bowie recorded when he lived in what was then West Berlin.

East Germany built the Wall, which it called an "Anti-Fascist Protection Rampart", in August 1961 to halt a mass exodus to the West.

On November 9, 1989, East German border guards, overwhelmed by large crowds, threw open the gates to West Berlin, allowing free passage for the first time since it was built.

At least 389 people lost their lives trying to escape East Germany, according to an official toll, although victims groups put the figure much higher.