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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An action shot of a chameleon as it snares its prey with its extended tongue has scooped top prize in an animal photography competition.
Arnis Zalkalns, a 41-year-old Latvian builder, vanished a week after 14-year-old Alice went missing from her home at Hanwell.
The 14-year-old vanished near her home in west London exactly three weeks ago.