The Queen has led the Remembrance Sunday service at the Cenotaph in London, as commemorations were held across the UK in honour of those who died in wars and conflicts.
Thousands of current and former military personnel joined the Queen, together with the main party leaders, who have laid wreaths.
Prince William had earlier attended a service of remembrance in Afghanistan.
The British Legion says it has sold a record 46 million poppies this year.
Commemorations have been given added significance this year by another 12 months of heavy UK military losses in Afghanistan.
This year also marks the 90th anniversary of both the Cenotaph and the Tomb of the Unknown Warrior, who was interred in Westminster Abbey.
Following the two minutes silence at 1100 GMT, the Queen was the first to lay a wreath at the Cenotaph, followed by other members of the Royal Family, Prime Minister David Cameron, Liberal Democrat leader Nick Clegg, and opposition leader Ed Miliband.
Leaders of Scottish, Welsh and Northern Ireland political parties also laid floral tributes, as did defence chiefs, and High Commissioners from Commonwealth countries.
About 4,500 ex-servicemen and women marched past the Cenotaph, led by veterans of the Korean War.
Some 2,500 British personnel took part in the service at Camp Bastion, which was also attended by Defence Secretary Liam Fox.
Dr Fox said: "I am extremely proud that Prince William and I could come to Camp Bastion to stand alongside the men and women serving in Afghanistan today.
"I pay tribute to them, and everyone who has served before them in this conflict and others on behalf of the freedoms that the British people enjoy."
Padre Andrew Earl of the Royal Engineers, who joined in the Camp Bastion remembrance, told the BBC that troops in Afghanistan greatly valued all the services that are taking part across the UK.
He added: "The moment of silence gives us as individuals, and collectively, the opportunity to stand with our coalition allies to remember our friends and colleagues who have fallen in this conflict.
"But also those who are still out on the ground in this conflict facing danger today. It also links us to members of our regiment who have fallen in previous conflicts."
Remembrance Sunday was also marked in other Commonwealth countries including Malta and Sri Lanka, as well as at British war cemeteries across the world.
The British Legion has made the theme of this year's Poppy Appeal the "Afghan generation" of the armed forces and their families.
A British Legion spokesman said demand for poppies had been so great that it had had to dip into next year's supply.
Elsewhere across the UK, HMS Ark Royal crew members led a parade as part of Glasgow's ceremony in George Square, while a service was held in Glasgow Cathedral. And a ceremony was held at the Stone of Remembrance on Edinburgh's Royal Mile.
In Belfast, tributes at the city hall were led by the Lord Mayor while in Cardiff a march by the armed forces and veterans organisations was followed by a service at the Welsh National War Memorial in Cathays Park.
A series of ceremonies are also taking place in Coventry to commemorate the 70th anniversary of a German bombing raid in World War II that devastated the city.
A remembrance service at Coventry Cathedral was attended by dignitaries including the German ambassador and the Mayor of Dresden.