Across the country, thousands of people will be marking 100 days to go. At Weymouth and Portland, the venue for the Olympic and Paralympic Sailing competitions, a giant sandcastle has been built featuring the Olympic Rings, Xinhua reported.
There will also be activities including have a go sessions and interactive video games at many Live Sites throughout the day on Wednesday.
Seb Coe, the LOCOG president, planted an oak tree in the Kew Garden with Jimmy Hunt, Secretary of State for Culture, Olympics, Media and Sport Jeremy Hunt, to celebrate the country's role in the birth of the modern Olympic movement. It's one of the 40 'Coubertin Oaks' oak trees to be planted at venues around the UK on Wednesday. The trees were grown from acorns taken from an oak planted in 1890 during the visit of the founder of the modern Olympic Movement, Pierre de Coubertin, to the Wenlock Olympian Games.
IOC President Jacques Rogge said in a message sent to the celebration site in Kew Garden, "Organizing the Olympics is a team effort and over the next hundred days, people from all walks of life and from all around the world will play their part in helping to make London 2012 a Games that will inspire a generation.
"From the athletes refining their technique, to the volunteers learning about their roles, to the torchbearers carrying the Olympic flame and the people supporting them as they go, each and every one of them is playing their part in making these a Games to remember.
"Around the world, the excitement is growing and expectations are high but I am confident that Britain and London will deliver a great sporting event and a warm welcome too."
Coe said, "With 100 days to go to the start of the Games, millions of people are getting ready to do the best work of their lives and welcome the world this summer. There is a groundswell of support and excitement, not just in the UK, but internationally as the final countdown to the London 2012 Olympic Games begins.
"Whether it's the competing athletes or people getting ready to join their communities in supporting torchbearers on the streets of the UK, the whole world is getting ready for London. Expectations are high, and we won't disappoint."
Hunt said, "The London 2012 Olympics and Paralympics are within touching distance now and there's something incredibly exciting about that. The country is getting ready to welcome the world this summer, and it's set to be an amazing celebration that we will never forget."