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
New figures show that lifeboat crews in London were the busiest in Britain in 2014. We look back at some of their most dramatic rescues.
Vlogging from her home in London she has become a YouTube sensation, so successful she has just published her first book.
Health union workers were due to walk out on Thursday in a dispute over pay.