On Sunday evening, the royal family attended a service in Westminster Abbey, where the Queen was joined by President of Germany Frank-Walter Steinmeier.
The Queen, dressed in purple, and Mr Steinmeier watched as flowers were laid at the grave of The Unknown Warrior, later shaking hands at the end of the service.
In Paris, President Donald Trump, Russian President Vladimir Putin and German Chancellor Angela Merkel joined French President Emmanuel Macron and more than 60 other leaders at a ceremony at the Arc de Triomphe a century after the guns fell silent.
Many of the leaders walked side by side along the Champs-Élysées in a somber, rain-soaked line as bells finished tolling. Mr Trump and Mr Putin arrived separately.
The leaders missed the exact moment to commemorate the armistice as they were running late.
In London, crowds began lining up in the Mall in central London from 9am, many wearing poppies and carrying wreaths
Big screens were erected so those waiting could view the Cenotaph service.
Big Ben, which has been silent since renovations to the Elizabeth Tower began in August last year, struck 11 o'clock to mark the hour the Armistice was signed.
The Queen watched the Whitehall service from the balcony of the nearby Foreign and Commonwealth Office.