Power was switched off at the buildings at 20:30 GMT on Saturday March 28th.
The WWF wanted to create a "visual statement" to show leaders of the world how important tackling climate change is.
Before the switch-off, Colin Butfield, campaign director for WWF, said: "We are hoping it will be quite a positive experience."
"People are embracing it in all kinds of different ways," he added.
Although the upcoming G20 Summit is expected to be focused on the current global economic downturn, world leaders are expected to discuss climate change at length.
It is hoped that UK Earth Hour will show the leaders of the global support for tackling the problem.
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