Royals and world leaders have gathered for ceremonies marking 100 years since Britain joined World War One.
The Prince of Wales and David Cameron attended a service in Glasgow, while the Duke and Duchess of Cambridge are in Belgium with many heads of state.
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.
‘Deadly days’
The day’s events began in Liege, Belgium, where 50 heads of state gathered for a service to mark the invasion of Belgium.
French President Francois Hollande said the country had been the first battleground of WW1 and had offered “solid resistance” in Liege.
“Deadly days” followed when French and British soldiers joined the conflict, he said.
Speaking to the gathered European leaders, Prince William said: “We were enemies more than once in the last century and today we are friends and allies.
“We salute those who died to give us our freedom. We will remember them.”
Meanwhile, the service at Glasgow Cathedral – attended by Prince Charles, Mr Cameron, First Minister Alex Salmond, and 1,400 others – heard from a number of Commonwealth figures.
The BBC’s Laura Bicker said Sunday’s closing ceremony of the Commonwealth Games was being discussed by many before the service – but the celebrations were “put to one side” as the congregation took time to “remember and reflect”.
Prince Harry unveiled a memorial arch in Folkestone, Kent – where a parade followed the route taken by millions of soldiers who marched to the harbour to begin their voyage to France in WW1.
The beautiful military cemetery at St Symphorien has been transformed.
Across the boundary fence, in what is normally empty farmland, a great grandstand has risen, overlooking the graves of the first and the last British soldier to die in World War One.
Cables snake around the gravestones of British and German soldiers, laid here side by side after the battles that raged around Mons on summer days in 1914.
St Symphorien has become an arena, overlooked by a worldwide audience, where Monday evening’s televised event will mark personal sacrifice and celebrate new friendships.
Under the pine trees, David MacCarthy had come to find the grave of his great-uncle, killed 10 days after arriving in Belgium, aged 23.
Standing in front of the headstone with his daughter, Mr MacCarthy said he was proud to be here on this anniversary.
The families who have travelled here from Britain and Germany share those sentiments.
The St Symphorien cemetery is of particular significance as an equal number of both German and British soldiers are buried there.
Among them lies Private John Parr, who was the first British soldier killed on the Western Front, and Maurice James Dease who was the first recipient of the Victoria Cross in WW1.
Last night they gathered to hear Hampden roar for the closing ceremony of the Commonwealth Games.
Today the leaders and representatives from across the Commonwealth stood in silence at Glasgow’s medieval cathedral. They were given a single poppy for their own personal act of remembrance.
Officers representing the armed forces walked through the aisles next to a child. Some were from the local Sunday school or the Scouts and Brownies. They held a candle of peace and hope. An act of remembrance to be taken on by the next generation.
And it was 16-year-old Kirsten Fell from Dunbar who had the final word. She read an essay written after a school trip to Flanders where she had been placed in front of a headstone of an unknown soldier and told to imagine his life.
“They told us we would change,” she told the congregation. “It will always be with me and nothing will be forgotten. I will remember my soldier.”
Between 22:00 and 23:00, the Royal British Legion’s Lights Out event will see households, businesses and public buildings across the UK turn out their lights to leave a single candle or light burning.
The event was inspired by the words of wartime Foreign Secretary Sir Edward Grey, who said on the eve of WW1: “The lamps are going out all over Europe; we shall not see them lit again in our lifetime.”
Tower Bridge, the Eden Project in Cornwall, the headquarters of the Football Association and the Imperial War Museums in London and Trafford, Greater Manchester, are among the other buildings taking part in the event.
As part of a remembrance project called the Big Picture, giant images of candles will be projected on to buildings including Battersea Power Station in London and the Royal Liver Building in Liverpool during “lights out” hour.
Earlier Prime Minister David Cameron told BBC Breakfast the outbreak of war was “an extraordinary day in Britain’s history”.
“When you think that almost every family, almost every community was affected, almost a million British people were lost in this war, it is right that even 100 years on, we commemorate it, we think about it and we mark it properly,” he said.
The Queen will not attend any of the major ceremonies, but will pay her respects at a service in Crathie Kirk near Balmoral in Scotland. The Duke of Edinburgh will attend a ceremony at Sandringham Church in Norfolk.
On Sunday, the French and German presidents commemorated the 100th anniversary of Germany’s declaration of war on France on 3 August 1914.