Post Office workers have staged a strike in a row over jobs, pay and closures amid warnings of further action unless talks are held over the future of branches.
The Communication Workers Union said they had "wonderful" backing for the 24-hour walkout from their members as well as strong support from the public.
The union called the strike in around 370 so-called Crown offices in protest at plans to close or franchise 70 branches, as well as over pay.
The Post Office said over 100 branches were open despite the strike and accused the union of ignoring the "harsh commercial realities" it faced.
Andy Furey, CWU national official, said: "Our members have sent a strong message to Post Office management today by solidly supporting strike action against closures, job losses, and pay. We hope the Post Office listens to the concerns of their staff. They can avoid further disruption by agreeing to negotiations to resolve this dispute.
"It's ironic that the Post Office has been trying to say offices remained open today when by their own plans they want to lose 76 Crown offices. That's 20% of the network and would hit communities badly. We won't let these go without a fight and ask again for the Post Office to meet us to resolve this. We're confident there is a way forward but this has to be by negotiation."
Union officials said they were prepared to call further industrial action unless the dispute is resolved.
The CWU said MPs and local politicians offered their backing to the strikers, adding that members of the public were also showing support. The union said hundreds of jobs will be affected by the proposals, which it warned would cause further problems for high streets already suffering from shop closures. The CWU added that staff had not received a pay rise for two years.
Kevin Gilliland, network and sales director at the Post Office, said: "We regret any inconvenience caused to customers by today's strike action. However, over 100 of our Crown branches are open today and the remainder of our 11,800 branches are unaffected. The CWU is ignoring the harsh commercial realities being faced by the Post Office and other retailers across the UK.
"Crown branches are currently losing £40 million per year and this is being subsidised by public money. This cannot continue. The Post Office is transforming its network to improve customer experience and in turn bring in new business. We are committed to the Post Office remaining a key part of UK high streets and our plans ensure this will happen."