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.
More top news
Detectives investigating an assault in Newham have released CCTV footage in a bid to identify the two suspects who attacked a 23-year-old.
Concern is growing for the welfare of a 28-year-old woman who has not been seen for more than two weeks.
A police officer in today's London Pride parade surprised his partner by proposing in the middle of the parade.