The Queen laid the first wreath at a ceremony at the Cenotaph memorial in London, which was attended by Prime Minister David Cameron, military chiefs, service personnel and thousands of spectators.
In Edinburgh, tributes were led by First Minister Alex Salmond, who joined Scottish Secretary Michael Moore and military staff at the Stone of Remembrance at the City Chambers in Edinburgh. Mr Salmond laid a wreath on behalf of the people of Scotland before attending a service at St Giles' Cathedral.
Thousands of ceremonies were held yesterday to mark Remembrance Sunday, which fell on Armistice Day.
Mr Salmond said yesterday was an opportunity for every man, woman and child in Scotland to reflect on the immense sacrifice so many have made to protect our way of life.
"This moment allows us to pay tribute to all of our servicemen and women, past and present, who have laid down their lives in defence of our country," he said.
"It is important that we also remember that today's commemoration is not simply about historical events but also about the sacrifices servicemen and women today continue to make."
At the Cenotaph, a two-minute silence was observed at the stroke of 11. Almost 10,000 ex-servicemen and women marched past the memorial to commemorate their fallen comrades.
There was warm applause from the crowd as the parade marched past the war memorial, inscribed to The Glorious Dead.
The Prince of Wales and Duchess of Cornwall earlier attended a ceremony in Auckland as part of their Jubilee tour of New Zealand. In Northern Ireland, the prime minister of the Republic of Ireland Taoiseach Enda Kenny attended the remembrance service in Enniskillen.
The visit came as the Co Fermanagh town marked the 25th anniversary of the IRA Poppy Day bomb attack, which claimed the lives of 12 people.