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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A yellow weather warning has been issued for London today and tomorrow with "thundery downpours" expected.
Police are appealing for witnesses after a crash involving a car and three pedestrians in Islington.
Five rugby league legends were immortalised in bronze today outside Wembley Stadium.