Prince Charles laid a wreath at the Cenotaph in Whitehall as the nation observed a two-minute silence.
The Prince of Wales was joined by Prime Minister Theresa May, Defence Secretary Gavin Williamson and other national figures.
Senior royals including the Duke of Cambridge, the Duke of Sussex, who laid a wreath at Twickenham yesterday, and the Princess Royal also paid their respects, as the Queen watched on.
Also, for the first time, a German leader laid a wreath at the Cenotaph.
President Frank-Walter Steinmeier performed the duty on behalf of his nation in an historic act of reconciliation between the two countries.
The First Sea Lord, Chief of the General Staff, Chief of the Air Staff and Chief of the Defence Staff General Sir Nicholas Carter laid wreaths at the Cenotaph on behalf of the Armed Forces.
765 military personnel were also in attendance at the Cenotaph.
Big Ben, which has been silent since renovations to the Elizabeth Tower began in August last year, striked at 11 o'clock to mark the hour the Armistice was signed and the start of the two-minute silence.
The end of the silence was marked by cannon fire and The Last Post sounded by the Buglers of the Royal Marines before the wreaths were laid.
Defence Secretary Gavin Williamson said: "To be at the Cenotaph on Remembrance Sunday is a privilege and provides us with the opportunity for reflection along with millions of people in countries that continue to be strong allies.
"The First World War touched communities across the globe and I commend all those who have helped us remember the First World War generation.
"We will never forget them or the sacrifice of thousands of British and Commonwealth troops who have given their lives in other conflicts."
Among the thousands paying their respects was Lieutenant Commander Sarah Bligh of the Royal Navy.
She said it was her grandmother's stories of the war that inspired her to join the forces.
"The thought of it being 100 years is really poignant. I've got a photograph of my great grandfather taken 100 years ago to celebrate them all coming back from the war," she said.
"It's incredible to be here 100 years on."
The agreement, which was signed on the 11th hour, of the 11th day, of the 11th month in 1918, signalled the end of the First World War.
5 million men from 30 countries fought in the First World War and more than 9 million fighting men were killed, one third of them through disease.
At least 750,000 soldiers killed were British and over 188,000 were from the British colonies.
Crowds lined the Mall in central London early on Sunday morning ahead of a "people's procession".
Wearing poppies and carrying wreaths, hundreds gathered from 9am to mark 100 years since the guns fell silent.
The march started at St James's Park before turning into Whitehall and then the Cenotaph.
Across the world, commemorations have been taking place to honour the fallen.
In Paris, nearly 70 world leaders gathered for a service outside the Arc De Triomphe.
Crowds across Australia fell silent to commemorate the centenary of the Armistice.
Australian Prime Minister Scott Morrison led a minute's silence as thousands gathered for a national service of remembrance at the Australian War Memorial in Canberra.