The bronze statue, which draws at least 1 million visitors every year, was created as a tribute to fairy tale writer Hans Christian Andersen. The author penned a story about a sea king's daughter who fell in love with a prince but had to wait 300 years before she could turn from mermaid into human.
The statue's international fame grew after its head was stolen in 1963. Since then, it has been vandalized, exploded and repeatedly repaired. It was also exhibited at the Shanghai World Exposition in 2010.
Friday's celebrations also included speeches, a children's concert, music and birthday songs.