It is expected to hamper mail collections and deliveries in the capital and having a knock-on effect across the country.
Dave Ward, deputy general secretary of the Communication Workers Union, said the strike would have "a massive impact" on postal services in London and warned that further walkouts were likely.
The CWU has called the strike in support of its claim for a bigger increase in the "weighting allowance" paid to postal workers in the capital.
The union wants all London postal workers to receive an annual payment of £4,000. Royal Mail is offering a £300 rise, which would give those in inner London £3,784 and those in outer London £2,667 a year.
The Royal Mail says postal workers in London will be £50 a week better off due to a proposed national pay deal and weighting allowance increase.
Adam Crozier, its chief executive, said there was "no more money to come" and said the union was out of touch with customers and its members.
"A strike won't generate more money. It will do exactly the opposite," he said yesterday.
The company said it would take what action it could to minimise disruption.
CWU members voted narrowly last week against industrial action over the proposed national pay award, but a ballot of the union's 28,500 London members had 11,417 supporting action over the London weighting issue, with 4,316 against.
