National Theatre head Rufus Norris and artist Jeremy Deller were behind a Somme commemoration project with men dressed as World War One soldiers.
The "modern memorial" involved about 1,500 voluntary participants appearing in public spaces across the UK.
Photos and reactions to the project quickly spread across social media, connected by the #wearehere hashtag.
Each carried a card with the name of the soldier they represented and his age - if known - when he died.
The Battle of the Somme, one of the bloodiest of World War One, began on 1 July 1916. All of the 1,500 men represented died on the first day of fighting.
More than one million men were killed and wounded on all sides during the five-month conflict, with the British suffering a total of nearly 60,000 casualties on the first day alone.
The project, entitled We're Here Because We're Here, was commissioned by 14-18 NOW, the UK's arts programme for the World War One centenary.
The day-long event - code-named Project Octagon - saw "ghost Tommies" appear at shopping centres, train stations and high streets and beaches.
The Royal Exchange in Manchester, Birmingham Repertory Theatre and the National Theatres of Scotland and Wales were among 25 organisations involved.
According to organisers, it "broke new ground in terms of its scale, breadth, reach and the number of partners and participants involved".
Turner Prize winner Deller said he wanted to make "a contemporary memorial... that moved around the UK with unpredictability".
Norris described the work as "a truly national piece of theatre" that offered "a powerful way to remember the men who went off to fight 100 years ago."
The volunteers, who were drawn from a range of professions, were aged between 16 and 52, reflecting the ages of the men who would have fought in the Somme.
Jenny Waldman, director of 14-18 NOW, said the project had given "hundreds of young people across the UK the chance to find out more about... the bloodiest day in British military history."
Comedian Dawn French was among those to post a picture on Twitter, asking: "Has anyone seen WW1 soldiers on their commute this mornin?!"
Many others have posted their own pictures of soldiers they have sighted in locations as far afield as Chester, Glasgow and Newcastle.
Norris and Deller are appearing live on BBC Radio 4's Front Row at 19:15 BST to discuss the project.
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