The action is part of a series of UK postal strikes
Thousands of postal workers across London have walked out for 24 hours in a row over jobs and pay cuts.
Members of the Communication Workers Union (CWU) began the strike from 0300 BST in delivery offices and collection hubs across the city.
The union is protesting against job and pay cuts which it says were imposed by Royal Mail "without consultation".
Royal Mail said it was "getting on with modernisation in an entirely fair and reasonable way".
The industrial action is part of a series of strikes around the UK and London.
It is the 11th 24-hour postal strike to take place in London since June.
A CWU spokeswoman accused Royal Mail of "imposing change without agreement".
She said Royal Mail was "cutting jobs, cutting pay, cutting overtime, cutting hours and at the same time forcing people to change the way they are working".
CWU divisional representative Martin Walsh said: "Royal Mail has introduced revision after revision, which is failing.
"[Workers] cannot clear their normal workload, let alone the backlog that's within the system."
The union said up to 20 million items of mail remain undelivered in London sorting offices due to the strikes, but Royal Mail said there were no more than five million undelivered items.
Royal Mail operations director Paul Tolhurst said: "We will be able to clear the backlog over two or three days if the CWU gets back to work as they plan to."
He rejected union claims that Royal Mail was "bullying" postal staff into accepting revised duties.
"We are being as absolutely reasonable as we can in every circumstance," Mr Tolhurst said.
"We are trying to work with our [staff] but they are being told not to talk to the management."
Union members in south-west London and Wimbledon are due to stage a further strike on Thursday in action expected to last until Friday night.
London-wide walkouts will continue until Saturday, when CWU members across England will take industrial action.