The Royal Family, politicians and veterans have paid tribute to those killed in battle at the service of remembrance at the Cenotaph in London.
The Queen led the ceremony by laying the first wreath
The Queen laid the first wreath of poppies, followed by the Royal Family, politicians, service chiefs and Commonwealth envoys.
A wreath of fallen autumn leaves was laid by the widow of a British officer killed in Iraq in 2003.
After the ceremony some 8,000 veterans marched past the monument.
Amid tight security, wreaths were placed at the Cenotaph in Whitehall by the Queen, the Duke of Edinburgh, the Prince of Wales, the Duke of York, the Princess Royal and the Duke of Kent.
The packed crowd of spectators and veterans taking part in the march-past were checked with metal detectors in preparation for the ceremony.
Prime Minister Tony Blair and New Zealand's premier Helen Clark also laid wreaths at the stone monument, followed by party leaders and high commissioners from more than 40 Commonwealth countries.
The commanders of the Army, Navy and Royal Air Force then laid their tributes.
A wreath of fallen autumn leaves was also placed at the Cenotaph by Iraq war widow Raqual Harper-Titchener, whose husband Major Matthew Harper-Titchener was killed in Iraq in August 2003.
British troops serving in Iraq and Afghanistan were also holding events.
As Big Ben struck, a single cannon shot signalled the start of the two minutes' silence.
Following the ceremony, some 8,000 veterans paraded past the Cenotaph to pay respect to all those who have lost their lives in conflict.
Thousands joined the ceremony to remember the fallen
Watching the service from a balcony at the Foreign Office was Prince William, with his stepmother the Duchess of Cornwall, the Earl and Countess of Wessex, the Duke and Duchess of Gloucester and Princess Alexandra.
Prince Harry, a cornet in the Blues and Royals, was attending a private regimental remembrance service with the Household Cavalry.
There were simultaneous wreath-laying ceremonies in Cardiff, Edinburgh and Belfast, with other ceremonies at local war memorials in towns and villages.
It is the first Remembrance Day since soldiers executed for cowardice and desertion during the World War I received pardons from the government.
And Janet Booth, the granddaughter of Pte Harry Farr, who received a pardon this year, told BBC News attending this year's service was particularly poignant for her family.
All those who campaigned to have their relatives pardoned would view the event as "a victory march as well as a remembrance parade", she added.
Armistice Day events
On Saturday, a two-minute silence and various events across the UK marked Armistice Day, the anniversary of the official end of World War I on 11 November 1918.
Tony Blair was joined by New Zealand Prime Minister Helen Clark
The Queen and Tony Blair attended a service for New Zealand's war dead in central London.
An event in Trafalgar Square, called Silence in the Square, was held, while families of some servicemen killed in Iraq held a separate ceremony at the Cenotaph.
Later, the Queen joined the Duke of Edinburgh and other members of the Royal Family for the Royal British Legion Festival of Remembrance at London's Royal Albert Hall.
More than one million men and women from Britain and the Commonwealth died in World War I between 1914 and 1918, and nearly 500,000 in World War II from 1939 to 1945.
The number of British service personnel who have died in Iraq since the start of hostilities in 2003 stands at 121.
Since 2001, 41 British Forces personnel have died while serving in Afghanistan.