Freya Murray is competing in the women's marathon in place of world record-holder Paula Radcliffe, who failed a fitness test.
Thousands of fans are expected to flock to central London to watch the women's Olympic marathon, which will see Mara Yamauchi and Freya Murray carrying Britain's hopes in place of injured world record-holder Paula Radcliffe.
Yamauchi, who finished sixth in the Beijing Olympic marathon of 2008, and Murray, who was called up when Radcliffe withdrew with a foot injury on the eve of the Games, are up against a strong field which includes Kenya's Mary Keitany, winner of two successive London marathons, and her compatriot Edna Kiplagat.
Other strong contenders include Russia's Liliya Shobukova, the second-fastest marathon runner in history behind Radcliffe, and 22-year-old Ethiopian Tiki Gelana, who broke her country's marathon record earlier this year.
The 26-mile course, comprising three laps, will start and finish on the Mall, in front of Buckingham Palace, and take athletes past a plethora of historic monuments, including St Paul's Cathedral, the Tower of London and Victoria Embankment.
The men's race will take place on exactly the same course on 12 August - the last day of the Games and a week after the women's race. Both events start at 11am.
Although access to the Mall is ticketed, access to the rest of the course is free. Notable vantage points include Victoria Embankment Gardens and the London Eye.
Elsewhere, 27 big screens are showing the action in London - in Hyde Park and Potters Fields, just a few minutes' walk from the course, as well as Hackney's Victoria Park, Walthamstow Town Square and General Gordon Place in Woolwich.
Outside London, a host of regional towns and cities will be beaming the action on to big screen - from Newcastle and Middlesbrough to Portsmouth and Weymouth. Each of the major regional cities - Liverpool, Birmingham, Manchester and Leeds - will have its own giant screen, offering the thrill of the crowd and the buzz of a live atmosphere.
Victoria Embankment Gardens
Victoria Embankment Gardens are among the best places to see the marathon. Athletes will run the full length of Victoria Embankment from Westminster Bridge to Blackfriars Bridge. The garden will provide a bit of extra space as well as some shaded repair in case of hot temperatures.
Nettleton Court, opposite the Museum of London, offers a dedicated disabled viewing area, while spectators looking for a panoramic view of the action could even take the London Eye, although this is likely to be extremely busy.
St James's Park
The Mall, where the race starts and ends, is accessible just to ticket holders. Nevertheless the track passes by Birdcage Walk, on the other side of the St James's Park, where spectators can cheer on their favourites.
The London Eye
With its amazing view and air-conditioned capsules, the London Eye is an ideal spot for an elevated view of much of the action. Lengthy queues and high ticket prices are the downsides.
For more information on where to watch the marathon live on big public screens, click here.