Organisers hope millions of people will participate across the UK.
Britain declared war on Germany on 4 August 1914.
World leaders at WW1 centenary events in UK and Belgium
At that ceremony, Prince William paid tribute to the soldiers who "died to give us our freedom".
The day concludes with a candle-lit vigil at Westminster Abbey and a "lights out" event around the UK.
The public can join the switch-off ending at 23:00 - the time Britain declared war on Germany in 1914.
About 17 million soldiers and civilians were killed between 1914 and 1918.
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