US First Lady Michelle Obama described coming to London for the Olympics as "a dream come true".

Mrs Obama said she is "beyond proud" to be leading the US presidential delegation at London 2012 which will include a ringside seat at the opening ceremony.

Away from the sports action where she will be cheering for Team USA, she will be fronting a series of fitness and activity initiatives aimed at tackling US childhood obesity.

"Leading our nation's delegation and travelling to London is truly a dream come true," Mrs Obama said of her three-day visit.

"If anybody had asked me when I was 10 or 11, or actually 40, whether I would have been doing this, I would have bet not.

"Some of my fondest memories when I was young and not so young involved watching the Olympics on TV and cheering on Team USA."

She is due at a Buckingham Palace reception hosted by the Queen for the heads of state and to meet Prime Minister David Cameron's wife Samantha.

Many Olympic and Paralympic athletes, including US gymnast Dominique Dawes and basketball player Grant Hill who both won gold at the 1996 Olympics, will be part of the US delegation.

Also heading to the UK is Republican presidential candidate Mitt Romney, who will meet David Cameron in No 10 on Thursday.

Romney will be joined in London by his wife Anne, whose dressage horse Rafalca will be competing in the Olympics as part of the US Equestrian Team.

Mrs Obama is set to host an event for about 1,000 US military children plus US and British students to try to get them interested and excited by the spirit of the Olympics.

David Beckham and Manchester United stars are also set to appear at the Let's Move! London event on 27 July.

So far up to 1.7m people have become more active this year as part of the campaign triggered by Mrs Obama's challenge to the US Olympic Committee and 10 of its governing bodies to help youngsters get active in their communities, according to US government figures.

Mark Ronson, Katy B and The Wanted will be among the acts, with SpongeBob SquarePants and the Power Rangers, who will be on hand to entertain the children.

Gymnast Nadia Comaneci, sprinter Carl Lewis and members of the US track and field team who will be competing at London 2012 are among the line-up of famous sporting faces set to attend.

It is hoped the sports stations provided by Nickelodeon, the NBA and USA Basketball, US Tennis Association, USA Field Hockey, USA Track and Field, Manchester United and Right to Play will help the youngsters get active.

The Buckingham Palace reception is later that evening on the day of the opening ceremony.

Mrs Obama meets Mrs Cameron the next day and will also get the chance to cheer on Team USA.

The delegation will also be spearheading 28 July as an Olympic fun day so that US youngsters can cheer on their athletes in London but also try out different sports in their neighbourhoods.

Describing it as "really cool," particularly as so many of the sports may be new to the children, Mrs Obama said: "So as our Olympic athletes begin to compete in London they will truly be inspiring a generation of young people to get active and to strive for excellence.

"They are going to be reminding us all that being an Olympian is not just about winning the gold or setting a new record - it is really about pushing yourself and believing in yourself and refusing to give up no matter what obstacles you might face.

"I am very excited about this trip for so many reasons but I am excited that it will serve as a powerful opportunity to send another message to the kids in this country and other countries about the importance of staying fit, learning to compete and staying healthy.

"This is not just about sports, this is about being active."

Her official duties, before departing on 29 July. will include touching base with the US Olympic Committee, the US Olympic alumni athletes and Team USA supporters.

Behind closed doors she will also find time to greet and thank members of the US Embassy and their families.