Crowds have begun to gather around the cenotaph in Whitehall ahead of the annual service to pay tribute to those who have died in major conflicts.
The Queen will lay a wreath in memory of those who lost their lives.
Prime Minister David Cameron and other party leaders will also leave tributes at the memorial.
I'll be laying a wreath at the Cenotaph this morning. We must never forget those who sacrificed so much for our country.
The Defence Secretary Philip Hammond has said Britain must never forget the ultimate sacrifice of the 446 servicemen and women who have died in Afghanistan, as the country prepares to pay tribute to those who have died in major conflicts.
Mr Hammond said this year's service in Camp Bastion, in Afghanistan will hold particular significance as it will be the last large-scale act of remembrance in the country before more troops are recalled ahead of the completion of operations at the end of next year.
"As we get closer to the culmination of this 12-year campaign, it is right to reflect upon the sacrifices that have been made by the brave men and women of our Armed Forces, and their significant achievements", he wrote in The Sun on Sunday.
"Four hundred and forty six British servicemen and women have made the ultimate sacrifice in Afghanistan on our behalf, putting their lives on the line to protect us. We should never forget them and we should never forget their families, who are forever without their loved ones."
Mr Hammond also said the dedication, professionalism and courage of Britain's armed forces was "truly the finest in the world".
Remembrance Sunday services are to be held today in tribute to members of the armed forces who have died in major conflicts.
The Queen will lay a wreath in memory of those who lost their lives, and will take part in a service at Whitehall's Cenotaph.
Other senior royals, Prime Minister David Cameron and other party leaders also leave floral tributes at the memorial.
After bereaved families of servicemen and women who lost their loved ones marched silently into the hall, a short video was played to the 4,000-strong audience, showing the mother of Lance Corporal James Ashworth, 23, speaking of her loss.
The soldier, from Corby, Northamptonshire, was awarded a posthumous Victoria Cross in recognition of his courage while serving with the 1st Battalion The Grenadier Guards in Helmand province last year.
The citation mentioned his "total disregard for his own safety" in ensuring a grenade was posted accurately in the battle against an insurgent.
There were tears in the royal box at the Royal Albert Hall as the Queen and Duke of Edinburgh joined thousands of people in commemorating the nation's war dead at the Festival of Remembrance.
The festival, organised by the Royal British Legion, featured stories of those who had served in conflicts past and present.
One particularly emotional moment came when the bereaved families of servicemen and women who lost their loved ones marched silently into the hall.
Labour leader Ed Miliband has said the Royal Legion Festival remembering the Armed Forces for Armistice Day is a "chance to honour" servicemen and women.
Very moving Festival of Remembrance tonight at the Royal Albert Hall. A chance to remember and honour the service of our Armed Forces.
The Queen and the Duke of Edinburgh have arrived at the Royal British Legion Festival of Remembrance as the nation begins three days of honouring those who have lost their lives in conflicts past and present.
Queen Elizabeth II and the Duke of Edinburgh arrived tonight to view the plaque commemorating the renaming of the South Steps at the Royal Albert Hall in London.
British troops serving in the Middle East marked Remembrance Day at a sunset ceremony at their Qatar base.
Deployed personnel serving with the Royal Air Force's 83 Expeditionary Air Group were among the first in the world to mark the date, due to the time difference.
More than 100 British and coalition servicemen joined the RAF's senior operational commander in the Middle East and Afghanistan for the service, where senior officers from the RAF, Royal Canadian Air Force, US Air Force and other coalition forces laid red poppy wreaths in memory of the fallen.