"The UK is pleased to support International Yoga Day.
"We were one of 177 countries to vote in favour of Prime Minister (Narendra) Modi's proposal and we are pleased to see the enthusiasm with which it is being embraced, both in UK and around the world," Cameron said in his message.
The UN adopted an India-led resolution declaring June 21 as 'International Day of Yoga' in December last year, less than three months after Modi proposed the idea.
The main event on the bank of the river Thames in London marked the launch of other smaller events across various cities this morning.
It was held at Bernie-Spain Garden, on the South Bank of Thames, and commenced with the lighting of a lamp followed by speeches and exercises and included a lecture by eminent ecologist and philosopher Satish Kumar titled 'Yoga - The Way of Life'.
The event was organised by the Indian High Commission and the Government of India Tourist Office in collaboration with the British Wheel of Yoga, Sivananda Vedanta Centre and Art of Living Foundation.