Hartlepool biggest post office was closed for the day as a result of industrial action.
The Crown office in Middleton Grange shopping centre was one of more than 100 branches that were shut nationwide on Thursday because of a strike by postal workers in disputes over jobs and pensions.
Customers were informed by a poster in the window advising them to go instead to one of five other branches in town.
Members of the Communication Workers Union (CWU) and Unite walked out in protest at the closure of offices, job cuts and changes to the pension scheme.
A total of 118 out of 305 Crown post offices were closed nationally.
But the Post Office said almost 99% of its 11,600 branches remained open despite the action.
Dave Ward, general secretary of the CWU, said the Post Office was at “crisis point”, and urged the Government to stop the “cycle of closures, job losses and attacks on workers’ terms and conditions”.
In Hartlepool, customers were advised to attend Post Office branches in Raby Road, Shrewsbury Street, Catcote Road, Brus corner at West View and Owton Manor Lane.
The unions also warned of further action, although talks aimed at resolving the row are planned next week.
Kevin Gilliland, the Post Office’s network and sales director, said: “We apologise to any customers who have been inconvenienced by the disruption to service in a very small number of branches.”
He added: “We are making steady progress to modernise the UK’s biggest retail network. These changes are needed to make our services better for customers and ensure that Post Office branches thrive at the heart of communities for future generations.
“In just over three years the Post Office has modernised over 6,000 branches, leading to improved facilities and more convenient and accessible services for customers.”
Mr Gilliland said the Unions’ comments regarding the business’s performance are misleading and stressed that people across the business continued to be kept fully informed of any changes that might affect them, including its proposals for changes to its Defined Benefits pension scheme.
He said the pension plan is being subsidised by the fund’s surplus but is due to run out next year.
He added: “Once this happens, the costs to the business of meeting existing commitments will significantly increase and will not be sustainable. We therefore need to close the DB plan before the surplus runs out.”