Britain declared war with Germany at 23:00 BST on 4 August, 1914.
Commemorations are being led by key figures across the globe taking part in services of remembrance.
People are encouraged to switch their lights out between 22:00 and 23:00 BST, with candlelit vigils being planned.
The Lights Out project was inspired by the words of wartime foreign secretary Sir Edward Grey.
On the eve of war, he said: “The lamps are going out all over Europe; we shall not see them lit again in our life-time.”
We will remember them.
