The Royal Mail says modernisation plans are being blocked
More than 600 postal workers in the Bristol area have walked out from two mail sorting centres.
The 24-hour action - triggered by an ongoing dispute over pay, jobs and conditions - is taking place at depots in Filton and Severn Beach.
Letters, bulk mail, Amazon online orders and flower deliveries will be hit by the action. Post on Friday is likely to be minimal.
Royal Mail has condemned the action by Communication Workers Union members.
A Mail spokesman said: "The union claims to support modernisation, yet acts to destroy it."
The Severnside depot handles bulk mail, Amazon orders and flowers for customers across the UK, while the Bristol Mail Centre in Filton handles all collections from post boxes and businesses in Bristol.
Next week, the union plans further action at Filton on Tuesday, which could lead to 1,100 mail sorters walking out, as well as a Wednesday stoppage at Severn Beach.
Post deliveries on Thursday are likely to be very light.
David Wilshire, secretary of the CWU's Bristol branch, said the local actions - part of a concerted series of strikes around the country - had brought the Royal Mail back to the negotiating table.
"The fact they [the Royal Mail] are making overtures means the pressure must be telling," he said.
A large-scale walkout at 19 mail offices in the West is being planned for the later part of next week, unless union demands are met.