The royals paid their respects to the fallen, taking part in events around the capital ahead of Remembrance Day.
Pop groups The Saturdays, Union J and Luminites launched this year's Poppy Appeal with a concert at RAF Northolt.
A group of girls - whose fathers all serve in the Armed Forces - are aiming to top the charts with the official 2013 Poppy Appeal single.
Prince Harry and the Duke of Edinburgh have opened the Field of Remembrance at Westminster Abbey in London.
The royals spent nearly an hour meeting veterans and members of the Royal British Legion and Poppy Factory as they were led around the 100,000 crosses that have been planted in memory of fallen soldiers.
After arriving at the Abbey in a Rolls Royce, the pair paid their respects by each laying a small cross of remembrance in front of two wooden crosses from the Graves of Unknown British Soldiers from the First and Second World Wars.
David Cameron has welcomed military personnel, celebrities and singing group The Poppy Girls to Downing Street ahead of Remembrance Day.
The Prime Minister tweeted: "Wonderful to hear the Poppy Girls singing at No 10 to help support this year's Poppy Legion appeal."
The Duke and Duchess of Cambridge stunned hundreds of commuters in London today as they joined poppy sellers at a tube station.
The royal couple travelled on a Routemaster bus to High Street Kensington station where they met military personnel and volunteers supporting London Poppy Day.
William and Kate were swamped by members of the public taking pictures as they walked along the high street to the station's entrance hall.
Some 2,000 uniformed personnel, veterans and their supporters are hitting the capital's streets today for the Royal British Legion appeal ahead of Remembrance Sunday.
The Royal Navy's HMS Lancaster has paid its respect to the fallen by wearing a poppy ahead of Remembrance Day.
Thousands of miles away from the UK, the ship's staff gathered on the flight deck and created a bright red poppy before holding a moment of silence.
The ship is carrying out patrols in the Caribbean but remembered those who gave their lives, or have been injured, in duty.
The Duke and Duchess of Cambridge will welcome military supporters into their palace grounds today to mark London Poppy Day.
The royal couple will meet staff and volunteers taking part in the Royal British Legion appeal who will call at Kensington Palace as part of a tour across the capital on a 1960's Routemaster Bus.
Prince Harry and his grandfather, the Duke of Edinburgh, will pay their respects at the Field of Remembrance in Westminster Abbey.
Each will lay a Cross of Remembrance in front of two wooden crosses from the Graves of Unknown British Soldiers from the First and Second World Wars.
The Last Post will sound, marking the start of two minutes silence, after which the two royals will visit meet veterans of more recent battles.
The Duke and Duchess of Cambridge are also expected to attend separate Remembrance events later today.
A thief has been caught on CCTV stealing a Poppy Day Appeal tin from a garage counter - by cutting a tie with scissors.
Unemployed Leon Cowley, 31, can be seen calmly chatting to the shopkeeper before stealing the tin when the store owner's back is turned.
He cuts the tie using a pair of scissors and slips it into an inside pocket before paying for a pack of mints at the Blackhorse Garage in Sheen Road, Richmond, Surrey, at around 10pm on Sunday October 27.
Cowley, of Chaucer Avenue, Richmond, who is wearing a grey and black striped jumper and black jacket in the footage, was arrested on November 1.
He appeared at Wimbledon Magistrates' Court today where he pleaded guilty to theft. He will be sentenced at the same court on November 13.
Prime Minister David Cameron has shown his support for this year's Royal British Legion Poppy Appeal by posing outside Downing Street this morning with the Poppy Girls.
The five-strong singing group, aged between 10 and 17, are the daughters of five men serving in the Armed Forces and have released a fundraising single for the Royal British Legion's appeal.
- The use of the poppy was inspired by the World War I poem "In Flanders Fields" by Lt Col John McCrae. Its opening lines refer to the many poppies that were the first flowers to grow on soldiers' graves
- In 1918, American Christian worker Moina Michael, inspired by the poem, published "We Shall Keep the Faith". She vowed to always wear a poppy and began distributing them at conferences
- Until 1996, poppies were made by disabled veterans in Canada, but have since been made by a private contractor
- A team of about 50 people - most of them disabled former British military personnel - work all year making millions of poppies at the Poppy Factory in Richmond
- To commemorate animal victims of war, Animal Aid has issued a purple poppy, which can be worn alongside the traditional red one