Thousands of people gathered to watch the display, which was also screened live on National Lottery Draw Show on BBC One.
Agency Imagination were brought in to install a new 360-degree LED information band at the top of the Tower, showing the number of days to the Opening Ceremony of the London 2012 Games.
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"We are well and truly on the road to London 2012, and BT is thrilled to be able to bring the nation together like this," said BT group marketing and brand director Suzi Williams.
"In our role as communications services partner, we want to inspire and engage people like never before - what better way to do this than returning the BT Tower to its original role as an information tower."
"I hope it's a taste of things to come in 2012 when the whole country will celebrate the Games coming to London," added LOCOG chairman Sebastian Coe.
"We are delighted that our sponsors are playing such a big role in exciting and inspiring the nation, whether through grassroots sports activation programmes or events like the BT Tower illumination this evening. It's great to have a world-class company, like BT, on board."
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