Pictures of the ceremonies held across the UK and in Belgium marking 100 years since Britain joined World War One.
The Queen was also among those to pay their respects in Scotland when she attended a memorial service at Crathie Kirk Church in Aberdeenshire.
The Duke and Duchess of Cambridge, Prime Minister David Cameron and Prince Harry attended a twilight ceremony at the St Symphorien Military cemetery, near Mons, Belgium. Mons was the location of a large battle during WW1.
Prince William and David Cameron laid loose flowers as a mark of respect at the ceremony - rather than wreaths or poppies, which became a tradition after the end of World War One.
Across the country lights were switched off and candles were lit, including outside Number 10 Downing Street, as people remembered all those who lost their lives during WW1.
The lights were also switched off at the Houses of Parliament, as part of the UK-wide "Lights Out" event. The day before the war began the then-foreign secretary Sir Edward Grey said: "The lamps are going out all over Europe; we shall not see them lit again in our lifetime."
A service was held at Westminster Abbey, in London, where a light installation called Spectra beamed a column of white light into the sky while a candle-lit prayer vigil was held inside.
Members of re-enactment group the Great War Society were showered by poppies in a ceremony at the Tank Museum in Bovington, Dorset, on Monday.
King Philippe of Belgium was met by a girl dressed in white in Liege. She carried a white balloon as a sign of peace.
The Duke and Duchess of Cambridge spoke to French President Francois Hollande at the ceremony.
Prince Harry laid a wreath and met members of the armed forces during a visit to Folkestone, Kent.
Also in Folkestone, balloons decorated with poppies and carrying the names of soldiers who died during the war were released into the sky.
The Tower of London moat has been turned red by 888,246 ceramic poppies, one for every British and Commonwealth soldier killed during the war.
100 white crosses have been placed outside New Zealand's parliament building in Wellington, in memory of the more than 18,000 New Zealand troops killed in WW1.