Hollywood actress Carey Mulligan and Olympian and British sprinter Adam Gemili are taking part in the Run to the Beat half marathon.
The race powered by Nike+ is London's biggest half marathon, with 19,000 runners set to make their way around the 13.1 mile course.
It starts and finishes in Greenwich Park.
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The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The family of the police officer who was murdered in the Westminster attack have thanked those who were with him 'in his last moments'.