Britain was plunged into darkness as lights were switched off for an hour across the country Monday night - all except one.
To mark the centenary of the First World War a single light was left burning in London as a powerful symbol of hope amid darkness.
Royalty, political leaders and relatives of the fallen united yesterday to remember the sacrifices and losses exactly a century on from Britain's entry into the First World War.
At 11pm on August 4 1914, Britain declared war on Germany, ushering in four years of darkness and despair. Until the armistice was signed on November 11 1918, soldiers engaged in the bloodiest conflict the world had known.
A spectacular sound and light installation at Victoria Tower Gardens called Spectra by Ryoji Ikeda pierced the London night sky above the Houses Of Parliament.
In a sign of respect of all that was lost, millions also took part in a powerful tribute to the fallen, switching off lights at their homes and offices to mark an era of appalling tragedy.
The nationwide event was designed to echo the words of then-foreign secretary Sir Edward Grey on the eve of the conflict: "The lamps are going out all over Europe; we shall not see them lit again in our lifetime."
A moving twilight ceremony at St Symphorien military cemetery near Mons was the highlight of Monday's events in the UK and Belgium, with the service featuring the gradual extinguishing of candles, with an oil lamp snuffed out at the grave of the unknown warrior at 11pm - the exact hour war was declared.
The royals stood alongside Prime Minister David Cameron and counterparts from countries including France and German at commemorative events yesterday.
Mr Cameron said the Great War, which claimed millions of lives, including 750,000 from the British and Commonwealth, was "unlike any other".
The Duke and Duchess of Cambridge will join Prince Harry at the Tower of London today to each plant a ceramic poppy in an art installation symbolising the thousands of lives lost during the First World War.
Today William, Kate and Harry will visit the dry moat at the Tower - the site where more than 1,600 men swore an oath to the crown in August 1914 after enlisting for war.
The first ceramic poppy installed as part of the Historic Royal Palaces artwork was planted last month, and the final will be laid on Armistice Day, November 11.
In total 888,246 ceramic poppies, one for each British and Colonial death during the war, will be installed by a team of 8,000 volunteers.
Profits from the artwork will be divided between six service charities including Help for Heroes and the Royal British Legion.