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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Police said they are "extremely concerned" for the welfare of a "vulnerable" mother and her two children who have gone missing in Bexley.
It is hoped the 7,400-mile journey will become a cost-efficient and time-saving way of transporting trade between China and the UK.
The Supreme Court is set to decide on whether disabled travellers are legally entitled to have priority use of wheelchair spaces on buses.