The servicemen and women who died fighting for Britain are being honoured at Armistice Day ceremonies across the country and the Commonwealth.
The Queen led the nation in a two-minute silence
The Queen and the Duke of Edinburgh each laid a cross to open the Field of Remembrance at Westminster Abbey.
At 1100 GMT, in line with tradition, they led the nation in a two-minute silence, before meeting veterans.
The Royal British Legion staged a flypast at 1800 GMT and dropped three million poppy petals above the Thames.
The petals - one for every British and Commonwealth service person killed in action since the beginning of World War I - were dropped between Tower and Westminster bridges by an original World War II Douglas Dakota DC3 aircraft during a two-minute flypast.
The river will be lit up in red every night until Sunday when the Queen, accompanied by about 9,000 veterans, will lay a wreath at the Cenotaph.
At the Field of Remembrance, relatives and friends will plant about 20,000 tiny wooden crosses, each adorned with a blood-red poppy, the name and rank of a fallen loved one and a message of commemoration.
The crosses are laid out in regimental order by the Royal British Legion.
Among the crosses, the occasional crescent or Star of David serves as a reminder Muslims and Jews have also laid down their lives for peace and freedom.
The Queen Mother first opened the Field of Remembrance in 1936.
For many years, she laid a cross in memory of her brother who was killed in World War I.
A Field of Remembrance was also opened in Cardiff by First Minister Rhodri Morgan and World War
II veteran Sir Tasker Watkins, 85, who won the Victoria Cross in 1944 as a major in
the Welsh Regiment and later became a privy counsellor, lord justice
of appeal and deputy chief justice of England.
Armistice Day has been a tradition in Britain since King George V issued a proclamation in 1919 that "all locomotion should cease, so that, in perfect stillness, the thoughts of everyone may be concentrated on reverent remembrance of the glorious dead".
Troops stationed in Iraq observed the two-minute silence
In 1918, on the 11th hour of the 11th day of the 11th month, the guns fell silent and World War I ended.
Troops stationed in Iraq were among an estimated 45 million British people who observed the two-minute silence.
At Oxfordshire County Hall, a short Armistice Day service was held before the opening of an inquest into the deaths of three Black Watch soldiers killed in a suicide bomb attack in Iraq on Thursday last week.
In Southport, Merseyside, the family, friends and colleagues of Major Matthew Titchener, 32, a Royal Military Police officer killed during an ambush on his jeep in Basra in August last year, attended a ceremony to unveil his name on the war memorial in his home town.
On Wednesday a delegation of families of those killed or still fighting in the current Iraq war laid a wreath of poppies on the doorstep of 10 Downing Street.
The 11-member group, from the newly-formed Military Families Against the War, included Rose Gentle and Reg Keys, who both lost sons in Iraq.
They said the wreath symbolised the "blood on the doorstep of Tony Blair".