The officers are protesting against proposed changes to their pay and conditions.
Some 20,000 officers from all 43 forces across England and Wales are thought to be taking part in the first police march in the capital for more than four years.
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Across the country people are pledging to run 3.7 miles in memory of Matt Campbell who died while taking part in the London Marathon.
Jonathan Munro is giving evidence during the latest stage of a High Court trial in London.
But while he wasn't showing it, PC Tyler O'Hare was enjoying every minute of it.