Commemorations are being held in the UK and France to mark the centenary of the start of the Battle of the Somme.
A ceremony at the Lochnagar crater on the battlefield began the day’s events, before a two-minute silence was held ahead of the exact moment on 1 July 1916 when the battle began.
On Thursday night, the Queen laid a wreath of flowers on the Grave of the Unknown Warrior in Westminster Abbey.
The battle saw more than one million men killed and wounded on all sides.
The Battle of the Somme, one of World War One’s bloodiest, was fought in northern France and lasted five months.
At a vigil in France, the Duke of Cambridge paid tribute to the fallen soldiers, saying “we lost the flower of a generation”.
The British and French armies fought the Germans in a brutal battle of attrition on a 15-mile front.
At the ceremony at the Lochnagar crater in La Boiselle, France, a rocket was fired to simulate the artillery fire, followed by whistles to symbolise those that were blown 100 years ago as men scrambled from the trenches.
Later, there will be another ceremony at the Thiepval memorial, near the battlefield in northern France, attended by 10,000 people including the Prince of Wales and Prince Harry and hundreds of descendants of the those who fought in the battle.
In Manchester, there will be a national service of commemoration, with a service at the cathedral at 15:00 BST and concert at 19:30 BST.
Leaders from Northern Ireland and the Republic of Ireland will attend a service at the Ulster Tower later.
At the Westminster Abbey Service on Thursday, the Queen was joined by the Duke of Edinburgh as she laid flowers at the Grave of the Unknown Warrior.
The tomb holds an unidentified British soldier killed on a European battlefield, brought back and buried in the abbey to honour the unknown dead of the war.
The Bishop of London, the Right Reverend Dr Richard Chartres, said the legacy should be that people worked towards reconciliation to ensure children never endured what the soldiers of World War One faced.
Society must strive to reach an accord and reject “those who would stir up hatred and division,” he said.
Prime Minister David Cameron, who also spoke at the service, his wife Samantha, and Labour leader Jeremy Corbyn were among other figures at the service.
The Duke and Duchess of Cambridge and Prince Harry paid their respects in France, attending a vigil at the Thiepval Memorial, located close to the battlefields of the Somme, near Amiens in the north of the country.
Prince William spoke of European governments “including our own” who failed to “prevent the catastrophe of world war”.
“We lost the flower of a generation; and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life,” he said.
“It was in many ways the saddest day in the long story of our nation.”
Prince Harry also spoke at the event, reading the poem Before Action, by Lieutenant WN Hodgson of the 9th Battalion the Devonshire Regiment, who wrote it days before he was killed in action on 1 July 1916.
Before the vigil, the three royals climbed to the top of the huge, newly renovated monument designed by Sir Edwin Lutyens to view the battlefield.
Some 70,000 British and Commonwealth soldiers with no known grave are commemorated at the memorial.
Earlier, the Dean of Westminster, the Very Reverend Dr John Hall, said: “As we imagine the feelings of those preparing for battle, the vigil will allow us to reflect on the cruel effects of warfare and to pray for lasting peace and justice in the world.”
At Westminster Abbey
By Nicholas Witchell, BBC royal correspondent
It was to Westminster Abbey that thousands went to pray during the desperate years of World War One.
It was to the Abbey that the body of one unidentified serviceman was brought back from the Western Front to be buried amid the greatest honour on 11 November 1920, two years to the day after the Armistice which ended the war.
And it was to Westminster Abbey that the Queen came to join a congregation in solemn remembrance of one of the most desperate moments of the war, the beginning of the Battle of the Somme.
There has been no more grievous day in the history of the British army, whose soldiers died in their tens of thousands as they advanced on the German lines.
In their remembrance – and in memory of all those who died – the Queen placed a wreath at the Grave of the Unknown Warrior. A bugle used by the Welsh Guards at the Battle of the Somme sounded the Last Post.
And an all-night vigil was mounted, to remember those young men who – 100 years ago tonight – waited in their trenches to face their destiny.
Welsh First Minister Carwyn Jones joined personnel from the Army, Royal Navy and the Royal Air Force for the start of a vigil at Llandaff Cathedral in Cardiff.
“Those who fought bravely for our futures should never be forgotten,” he said.
In Scotland, an overnight vigil was held at the National War Memorial.
A whistle, which was sounded to lead men over the top, will be blown by Scots soldier Alan Hamilton at 07:30 BST to mark, to the minute, 100 years since the battle began. The whistle belonged to his great uncle.
And in Northern Ireland, a vigil is being held at the Somme Museum near Newtownards, County Down. A guard of honour, including serving soldiers, was present throughout the night.
The Prince of Wales and Duchess of Cornwall, and 10,000 members of the public chosen by ballot – including hundreds of schoolchildren – will also attend a service of commemoration on Friday.
The royal couple will then attend ceremonies for Northern Irish and Canadian victims of the battle at the nearby Ulster Tower and Beaumont-Hamel Newfoundland Memorial, respectively.
The Duchess of Cornwall will lay a wreath at the grave of her great-uncle, Captain Harry Cubitt, who was killed on the Somme in September 1916 while serving with the Coldstream Guards.
He was the eldest, and the first, of three brothers to die serving on the Western Front.
The Battle of the Somme
- Began on 1 July 1916 and was fought along a 15-mile front near the River Somme in northern France
- 19,240 British soldiers died on the first day – the bloodiest day in the history of the British army
- The British captured just three square miles of territory on the first day
- At the end of hostilities, five months later, the British had advanced just seven miles and failed to break the German defence
- In total, there were more than a million dead and wounded on all sides, including 420,000 British, about 200,000 from France and an estimated 465,000 from Germany
Find out more:
- How the Battle of the Somme unfolded
- Why was the first day such a disaster?
- Timeline: World War One 1914-18
- Has history misjudged the generals of WW1?
- How did so many soldiers survive the trenches?
- WW1 centenary – full coverage
The Battle of the Somme was intended to achieve a decisive victory for the British and French against Germany’s forces.
The British army was forced to play a larger than intended role after the German attack on the French at Verdun in February 1916.
Among the worst hit were the “Pals” battalions, volunteer units of limited fighting experience.
Many were told to walk slowly across no man’s land, resulting in massive numbers of dead as they headed straight into German machine-gun fire.
The 2,000 men of the 1st and 2nd Bradford Pals, both part of the West Yorkshire Regiment, suffered 1,770 casualties in the first hour of the offensive as they attacked the heavily fortified village of Serre.
World War One finally ended in November 1918.