Millions of people have held a two minute silence to remember those who have died in wars.
Big Ben, which hasn't sounded since August, chimed its trademark bongs at 11am.
Elsewhere in the UK, events are taking place to mark the 99th anniversary of the end of World War One.
Armistice Day, which is also known as Remembrance Day, marks the day World War One ended, at 11am on the 11th day of the 11th month, in 1918.
Tomorrow on Remembrance Sunday, politicians and members of the Royal Family will gather at The Cenotaph in Whitehall, London, for a memorial service.