The traditional big tree on City Hall Square has hundreds of lights
Copenhagen is hoping cyclists will ensure the city has one of the most environmentally-friendly Christmas trees in the world this year.
Volunteers must use pedal power to produce enough electricity to make the hundreds of lights on the tree sparkle.
They have until 18 December to help power the tree, on City Hall Square, using this alternative fashion.
The event starts a week before the Danish capital hosts the UN Copenhagen Climate Summit.
During the summit, organisers say the city hall square will be take on the name "Hopenhagen Live" and become a hub for environmental and cultural activity.
They hope this alternative means of lighting the large tree will prevent the regular nine tonnes of carbon dioxide from being produced.
Project manager Stephen Risguard said that a number of bikes are connected to a different section of the tree and if no-one used the cycles, then there would be no power to turn the lights on.
Pia Allerslev, the city's mayor responsible for culture and sports, said the tree would be the "greenest" that had ever lit up the square.
The city has more than 350km (217.48 miles) of cycle lanes and a reputation for being "bicycle-friendly".
Leaders of Britain, Germany, France, Spain, Australia, Japan, Indonesia and Brazil are all expected to attend the UN summit, from 7 to 18 December.