Tourists walk past one of the most famous paintings of the East Side Gallery as renovation works start in Berlin in October 2008. (Herbert Knosowski/Associated Press)

Artists who created murals on the Berlin Wall have begun restoring their open-air artworks, ahead of planned upcoming celebrations marking 20 years since the Cold War-era barrier fell.

The murals, which were painted on a 1,300-metre concrete stretch of wall running along the Spree River, are a major tourist attraction. However, over the years, the paintings have been subject to damage from weather, pollution, vandals and even tourists wanting to break off a souvenir to take home.

The murals have been dubbed the "world's largest open-air art gallery" by a group of artists known collectively as the East Side Gallery. According to gallery head Kani Alavi, four of the original artists have begun restoration work and 82 others have pledged to participate.

"I am excited and am particularly happy that the wall will be restored like a historical monument," Rose-Marie Schinzler, who painted two of the works, told Agence France-Presse.

However, there has been some controversy about the restoration efforts from some of the original artists.

Russian artist Dmitri Vrubel, responsible for painting the image of East German leader Erich Honecker kissing his Soviet counterpart Leonid Brezhnev, told German media that he refuses to paint the same image.

Another artist, Bodo Sperling, says he represents about a dozen others who are refusing to take part because they feel they have not received a fair share of the East Side Gallery's earnings over the years.

The restoration project is slated for completion in time for the 20th-anniversary celebrations in November.

East Germany's Communist regime erected the Berlin Wall in 1961 to stop a wave of citizens leaving for the democratic western half of the city. It stood for 28 years.