Madonna Harris before the Mongol Derby.
The 16 competitors must complete the race within 10 days, changing their native horses at specially set-up stations at intervals of no more than 40km.
Its promoters say the derby celebrates the horseback messenger system used by the warrior Genghis Khan.
GPS tracking indicates Harris holds a narrow lead over Justin Nelzen, from the United States, and Saskia van Heeren, from South Africa.
There is then a gap back to the next riders, led by Alisa Vermay, from Australia.
The derby, being run for the second time, is organised by a British company, The Adventurists, in collaboration with the Mongolia-based Tengri Group.