Sir Keir Starmer was among those to pay their respects to the fallen in an Armistice Day two-minute silence.
Poignant services were held across the country in locations such as the National Memorial Arboretum in Staffordshire, Westminster, and North London.
The Duke and Duchess of Gloucester attended the National Memorial Arboretum Armistice Day Service in Staffordshire and services were held in London, Edinburgh, Belfast and Portsmouth.
Foreign Secretary James Cleverly, who said Russia had “shattered” peace by waging war in Ukraine, was in Paris to attend a remembrance service hosted by French President Emmanuel Macron.
And Labour leader Sir Keir marked the moment at Euston Station, in central London.
Flag bearers, veterans and members of the public observe a two minute silence an Armistice Day service
Andrew Matthews
Members of the public observe a two-minutes silence in Westminster.
Jonathan Brady
Members of the public fall silent at Lloyd's in London.
Victoria Jones
Conservative MP Michael Fabricant at the National Memorial Arboretum, Alrewas, Staffordshire.
Jacob King
Veterans and members of the public attend an Armistice Day service in Portsmouth.
Andrew Matthews
Mr Cleverly said: “Since 1918 we have marked Armistice Day and paid tribute to the brave men and women who have served to give us peace.
“Yet as we salute our troops this year, this peace has been shattered by a Russian aggressor.
“As we honour the war dead of the past, we also remember Ukraine’s fight for freedom today.
“The UK stands steadfast with our friends and allies in defence of freedom and democracy in Ukraine and I am proud to stand shoulder to shoulder with an historic ally in Paris today.”
The Met Office said there were “exceptionally mild” conditions across the UK as it predicted that Friday could be the warmest Armistice Day on record.
Temperatures across the four nations are in the mid-teens.
Sir Keir Starmer was among those to fall silent.
PA