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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Celebrations to mark St George's Day have started in Trafalgar Square with feasts and tea to be found in abundance.
People took to the streets of Brixton today to march against what they see as the 'gentrification' of the area.