The lights on some of Britain's most famous landmarks have been switched off for an hour tonight to remember the 17 million who gave their lives in the First World War.
Tower Bridge, the Houses of Parliament and St Paul's Cathedral all went dark from 10 pm.
Britain officially declared war on Germany at 11pm on August 4, 1914. The conflict lasted four years.
Lights across Britain have been switched off for an hour tonight in a tribute to the 17 million killed in the First World War.
Landmarks including the Houses of Parliament, Tower Bridge and St Paul's Cathedral all went dark from 10pm.
Prime Minister David Cameron had asked Britons to switch off all but a single light in their homes for an hour.
Britain's military personnel have spoken at their pride at commemorating their predecessors who fought in the First World War.Read the full story ›
A single candle has been placed outside Number 10 ahead of the Lights Out campaign.
The 14-18 NOW Lights Out campaign hopes people will leave a single light on between 10 and 11pm to mark the exact hour when Britain declared war on Germany.
Number 10 is among the buildings that will turn their lights out in an hour's time to mark the moment Britain declared war on Germany and joined the First World War.
Are you joining in the Lights Out event? Post pictures of your "moment of reflection" on our Facebook page.
A lone bugler played The Last Post during a moving ceremony at St Symphorien military cemetery near Mons, Belgium.
War must always represent the abject failure of humanity, the head of the Anglican church in Ireland has said.
Archbishop of Armagh Dr Richard Clarke said commemorating the First World War could not be spiritually separated from the situation in Gaza and other recent conflicts.
Dr Clarke said: "War must always represent the abject failure of the human spirit and of humanity itself.
"It can never be other and we should never pretend it is other."
The Duke of Cambridge and David Cameron have laid wreaths during a twilight ceremony commemorating the First World War.
People should "never fail to cherish" peace, David Cameron said as he joined a twilight ceremony to mark 100 years since Britain joined the First World War.
Cameron told those assembled at St Symphorien military cemetery near Mons, Belgium: "This was a war with an immense human cost - and we must always, always remember that, no matter how busy things are.
"In shaping that future, it is vital that we look to the past. Here on the continent of Europe we saw not the war to end all wars, but the precursor to another desperate and violent conflict just two decades later.
"We should never fail to cherish the peace between these nations and never underestimate the patient work it has taken to build that peace."
The Queen has attended a service of commemoration for WW1 soldiers in Aberdeenshire's Crathie Kirk Church.