Live from Table Mountain began at 0400 GMT and will continue to 1800 GMT, to coincide with the rising of the sun over the Indian Ocean and sunset over the Atlantic.
Hosted by Ben Malor of the World Service's Africa Live and Heather Payton of Outlook, the programme is being webcast live in video and audio.
The programme is the most ambitious outside broadcast attempted by the World Service and will be the highlight of two weeks of anniversary events.
Celebrations have already included a five-hour live Global Party, featuring five concerts in cities around the world.
UN Secretary-General Kofi Annan gave a birthday lecture and celebrities from around the world talked about how they had been affected by the World Service.
Table Mountain was chosen for the central anniversary day event because it was the site of one of the World Service's first broadcasts.
Phil Harding, World Service director of English regions and news said: "The original broadcast from Table Mountain was an early example of collaboration between the service and other radio stations, something that remains hugely important today."
Top of the world
Highlights for the marathon programme will include interviews with Archbishop Desmond Tutu and Hollywood actress Angelina Jolie, who is also a goodwill ambassador for the UN.
Throughout the programme there will be features from around the world, from Moscow in Russia to Bombay in India.
A hang-glider will be interviewed over Rio and Turkey's leading novelist Orhan Pamuk will talk about his career as part of the festivities.
Other anniversary events on Thursday will include a live edition of Talking Point with World Service director Mark Byford.
Anniversary programmes then come to a musical finale on 21 December as DJ Steve Wright unveils the World Service's ultimate top 10 on his show.
The chart will have been compiled based on votes sent in by World Service listeners since the search to find the "World's ultimate number one" was launched on 9 November.
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