Deliveries to parts of the capital were halted and letters bound for other areas of the country were held up.
The Post Office said it was doing its best to keep mail moving, but a spokesman admitted: "Over a large area of the capital there is no mail service."
Pillar boxes across London were sealed because of the walk-out, which was set to continue disrupting services over the weekend.
The strike went ahead despite an injunction obtained by the Post Office against illegal sympathy action. The dispute started at the north-west London sorting office in Camden where workers are protesting at the introduction of a new computer system, which they complain has dramatically increased their workload.
Sympathy strikes spread to other sorting offices across the capital and by the end of the day, two-thirds of the workers were out.
A spokesman for the Union of Communication Workers said: "The prime objective for the union and its members is not only to keep within the law but to seek a speedy resolution to this dispute."
Both sides hoped to hold talks over the weekend.