Alesha Dixon and Pixie Lott spoke of the "absolute honour" of performing at the launch of the Royal British Legion Poppy Appeal.
The singers, the faces of the Poppy Appeal 2012, were cheered by crowds in London's Trafalgar Square as they headlined a free concert to launch the Legion's annual fundraising drive.
"It is such an honour to be performing here in Trafalgar Square," Pixie said before performing hits including Mama Do and It's All About Tonight.
"Thank you for coming down for such a great cause as the Poppy Appeal. It is a privilege to be here," she added.
Britain's Got Talent judge Alesha said it was an "absolute honour" to perform. She joked with the crowds as she performed hits including The Boy Does Nothing and Breathe Slow. "If it gets really rock 'n roll feel free to jump in the fountains because that always works," she quipped.
The concert featured military bands The Corps of Drums of the 1st Battalion Coldstream Guards, the 1st Battalion Irish Guards and the trumpeters from the Band of the Grenadier Guards.
Pixie also paid tribute to her grandfather, 84-year-old Charles Martin, who was in the Royal Navy, and had travelled to Trafalgar Square for the event.
The 21-year-old said she had paid a visit to RAF Northolt to see forces' families. "It was amazing, I have never done anything like that before, to meet the soldiers and their families and the children - and just the sacrifices they make and how important the money raised is to help them."
Speaking afterwards, Alesha said: "I think this appeal is important for all of us, it is the one thing that the whole country should come together and support because the soldiers are out there fighting for us, it is a massive, massive sacrifice and I want to say thank you to them."
The appeal's £42 million target is up on the record £40 million raised last year. People wishing to donate can do so at any time online or by text. Poppies go on sale from Saturday until November 11.