More than 2,500 postal workers in Scotland have joined a nationwide strike in a dispute over pay, pensions and future industry changes.
Royal Mail said it was disappointed at the scheduled strikes
The Communication Workers Union (CWU) said its 24-hour walk out would hit Thursday's deliveries, with further disruption planned at the weekend.
Staff at Scotland's main mail processing centres walked out at 1900 BST.
The Royal Mail said it was "hugely disappointed" at the strike action.
The strikes will see each section of the company, from the sorting and collection centres to those who make the deliveries, walking out for two separate 24-hour strikes over two weeks.
The move is designed to cause continuous disruption to the nation's postal service throughout the period at a "minimum cost" to CWU members, union officials said.
The union said the action reflected its growing frustration with the Royal Mail for its failure to take worker demands for job protection and increased pay "seriously".
The CWU fears 40,000 jobs will go as a result of increased mechanisation of the system.
Norrie Watson, divisional representative for the CWU in Scotland and Northern Ireland, said: "We accept that all modernisation will cost jobs but we want to do it in a negotiated manner.
"We have always been up for change in this business. It has changed, with the assistance of the union, for the past four to five years and we have had to suffer job losses in the region of 30,000 to 40,000."
The union is also objecting to a 2.5% pay offer.
Mr Watson said: "A postperson who goes out in all sorts of weather and delivers a very efficient service is entitled to a normal pay rise."
Ian McKay, director of Scottish affairs at Royal Mail Group, called for "realism" and said the dispute was the "last thing" that he wanted.
He said: "The CWU don't seem to believe that we are in the position that we are.
"We need to continue to change if we are to continue to compete in our new commercial situation."
Mr McKay said the dispute was about change not pay.
"If our competitors are 40% more efficient than we are, that is not because we have bad workers, it is because we have not got the machinery and the back-up to let our workers work to their best capability," he said.
Two 24-hour walkouts have been held in the last month - the first national industrial action at the Royal Mail for more than a decade.