Members of the public are welcome to join GLA staff, and representatives from different businesses, community organisations and faith groups.
There will be a selection of congregational Christmas carols, a variety of biblical and secular readings/poems and guest choirs performing a selection of songs.
This year's service focuses on volunteering and Team London, the Mayor's programme to increase volunteering in the capital.
Young volunteers aged between 18 and 25 from the City Year London's Corps will give readings.
The Choir of St Luke's Chelsea and the London Ambulance Service's choir The LA Singers will perform.