The Queen was leading ceremonies on Sunday to remember the men and women who have given their lives in conflict
The commemorations this year are especially poignant as they mark the centenary of the outbreak of World War One and the 70th anniversary of the D-Day landings.
A wreath-laying ceremony at the Cenotaph will be attended by the Queen, political leaders and veterans.
In a statement, David Cameron said: "Today we stand united to remember the courageous men and women who have served our country, defended our freedoms and kept us safe.
"We remember all those who have fallen and those who have risked their lives to protect us.
"We owe each and every member of our armed forces and the families who support them a tremendous debt - one that can never be repaid - and I pay huge tribute to their bravery and resolve."
The Queen was among the guests a tthe Royal British Legion's Festival of Remembrance at the Royal Albert Hall in London on Saturday.
She has also been among the thousands of visitors to the World War One poppy installation at the Tower of London. The display, entitled Blood Swept Lands And Seas Of Red, will remain in place until November 12.
Churches will also be marking Remembrance Day in their services and a service was to be held at the National Memorial Arboretum in Staffordshire.
The Bishop of Southampton the Right Reverend Jonathan Frost said: "We owe a great deal to the men and women who dedicate their lives to defending our country, sometimes paying the ultimate sacrifice. We will remember them before God and my prayers will be for their families on Remembrance Sunday."
