Members of the Communication Workers Union (CWU) walked out, hitting deliveries of post across the country.
Talks between the two sides were held on Thursday but there was no sign of any progress being made and the union is planning to step up industrial action in the coming weeks.
The union said the walkout by around 115,000 of its members is the biggest strike of the year amid long-running disputes in other sectors.
A further 19 days of strikes have been announced on different days throughout October and November in a major escalation of the dispute.
CWU general secretary Dave Ward said: “This is a significant announcement, but it is one which matches the level of anger our members feel at the way Royal Mail Group has treated them.
“The chief executive of Royal Mail Group is treating postal workers as if they are stupid.
“These are the same people that have kept the country connected and returned Royal Mail Group to record profit.
“Postal workers across the UK now face the fight of their lives to save their jobs and the service they provide to every household and business in the UK.
“We call on everyone to stand with their local postal worker.”
A Royal Mail spokesman said: “The CWU can be in no doubt of the impact its reckless pursuit of 19 days of industrial action has on our weakened financial position and the job security of its members.
“Royal Mail is losing £1million a day. We operate in a competitive market, and our customers have choices. Continued strike action will force our customers to make those choices sooner rather than later.
“Our invitation to enter into talks through Acas remains open. Our people need the CWU leadership to recognise the reality of the situation Royal Mail faces as a business, and to engage urgently on the changes required to adapt to customer demands in a highly competitive market.
“We apologise to our customers for the inconvenience the CWU’s continued strike action will cause.
“We are doing all we can to minimise any delays and keep people, businesses and the country connected.”
Royal Mail workers are on strike
Rui Vieira
Picket lines have been mounted outside Royal Mail delivery and sorting offices
Steve Parsons
