Norwegian Prime Minister Jens Stoltenberg has switched on the lights on the Christmas tree in Trafalgar Square, central London.
The tree in Trafalgar Square is a traditional gift from Norway
Since 1947 the pine tree in the square has been a gift from the people of Norway in recognition of Britain's support during World War II.
The tree was selected from a forest near Oslo in November and was then shipped across the North Sea.
The 70ft tree, more than 100 years old, is decorated with 500 lights.
The traditional Norwegian light decorations will be lit from noon until midnight every day until 4 January.
After the festive period the tree will be recycled, Westminster council said.
The Lord Mayor of Westminster, Carolyn Keen and the mayor of the Oslo are jointly hosting the event.