Thousands of postal workers in London have launched a two-day strike in a row over jobs and services.
Up to 8,000 members in London and about 160 in Scotland walked out from 0600 BST on Friday, according to the Communication Workers Union (CWU).
The union has accused Royal Mail of cuts which break a national agreement and threatened modernisation.
Royal Mail said it had honoured existing agreements and urged the union to call off the strikes.
A union spokeswoman said the strikes had been "solidly supported", with workers in London staging a protest outside the Royal Mail's new head office in Blackfriars.
Deputy general secretary of the CWU Dave Ward said: "We are now seeing cuts but not modernisation in the postal industry and there's only so long before this is going have a major impact on services.CWU's Dave Ward wrote:We are now seeing cuts but not modernisation in the postal industry and there's only so long before this is going have a major impact on services
"The CWU does not and has not blocked change.
"We are offering Royal Mail and the government a three month no-strike deal if they fulfil the agreement to engage the CWU over modernisation and move to get the company on a sound footing for the future."
Royal Mail said the three largest offices in London were not involved in the action so postal services would not be disrupted in many areas of central London.
Royal Mail apology
It added that 160 distribution drivers in Edinburgh were involved in Friday's action, and said contingency plans meant there would be no disruption to mail services or customers.
More than 300 are expected to take action at seven delivery offices in Edinburgh, Dunfermline, Cowdenbeath, Grangemouth, Bathgate and Irvine on Saturday.
Management volunteers have helped deliver post during the strike in London.
A Royal Mail spokesman said: "Royal Mail apologises to its customers for any disruption to the service caused by the strike action and assures them that we are doing all we can to deliver the mail.
"We have urged the union repeatedly to honour its public statements and not to call our people out on strike action which can only hurt our customers and damage our drive to build a strong and sustainable future for Royal Mail and its people - and to protect the universal service in the competitive marketplace."