Support for the first national postal strike in a decade has been "overwhelming", according to unions.
Union members voted overwhelmingly for strike action
However, Royal Mail said nationally up to 60% of its staff were working, with up to 22% at some London sites, while all but 26 post offices opened.
Communication Workers Union (CWU) members began the 24-hour walkout from 0300 BST after talks failed.
Workers rejected a 2.5% pay offer and said they were worried that modernisation plans would see jobs cut.
Royal Mail argues that changes are essential in order to compete with rival mail firms.
Picket lines have been set up outside sorting and delivery offices.
Unions have argued that Royal Mail's plans for modernisation would lead to 40,000 job losses, and have threatened further strikes.
The CWU's deputy general secretary Dave Ward, hailed the "overwhelming" support for the action, adding that the union had tried to reach an agreement but that Royal Mail were "refusing to negotiate".
"What Royal Mail are doing is not modernisation," Mr Ward said.
"The truth is, they are intent on cutting services, cutting jobs and cutting pay."
CWU members voted for the strike action with a majority of 77% on a turnout of 60%.
Royal Mail said it needed to change the way it operated after the UK's postal market was opened up to competition.
It recently lost a "critical" £8m contract to collect second class mail from online retailer Amazon, and blamed higher costs caused by the company's "failure to modernise operations" for the loss of business.
Earlier this year, Royal Mail was ditched by the Department for Work and Pensions in favour of UK Mail.
Businesses have complained that the strike may hurt operations
A spokesman said it was "very sorry" about the disruption to mail services, and that volunteers from management were trying to collect mail from Post Office branches and were also delivering Special Delivery mail.
Changes were needed so that the firm could "compete successfully against competitors".
"They are winning an increasing share of the mail market, not because they work harder than Royal Mail's people - but because technology has made them more efficient.
"The challenges won't go away and the longer we put off change, the bigger the challenge Royal Mail and its people face."
Industry watchdog Postwatch has called on people to avoid sending mail on Friday, fearing the system will struggle to cope when normal service resumes.
"There's going to be no collections and no deliveries. Most big post offices are going to provide a very, very tiny service if at all," the group's chair, Millie Banerjee told the BBC.
"On behalf of customers I have to say this is a very sad day. It is going to be hugely disruptive.
"We are just very disappointed that Royal Mail and the workforce haven't been able to get together and hammer out an agreement."
Companies said that they were worried it would take time for postal services to recover after the strike, hurting their operations.
Matthew Knowles, a spokesman for the Federation of Small Businesses, said that: "We are very disappointed that this strike went ahead."
"The livelihoods and jobs of many people in small businesses will be put at risk if this dispute continues for much longer," he added.
Mr Knowles warned that vital documents and payments to small firms could be delayed.