Sunderland's main post office was closed this morning after Royal Mail staff walked out as past of an ongoing dispute.
Members of the Communication Workers Union and Unite staged action on one of the busiest days of the year, mounting picket lines across the country - including outside the Fawcett Street branch.
The unions are embroiled in a long-running row over the franchising of Crown Post Offices - the larger branches usually sited on high streets - plus job-losses and the closure of a final salary pension scheme.
Dave Ward, general secretary of the CWU, said: "The Post Office is now at crisis point. It needs a new strategy as a matter of urgency."
Unite union officer Brian Scott said: "We believe that Saturday is the day when most people will be dispatching their cards and parcels to their relatives and friends abroad.
"We are taking this action because the management refuses to talk in a constructive manner about the pension scheme which is currently in surplus to more than £143 million.
"This is the retirement income of our members which is at stake and we are not going to stand idly by and let them lose thousands of pounds when they retire.
"More generally, it appears that it is only the unions that care about the future viability of the Post Office and the services it provides for communities across the UK.
"The management seems to have abdicated its responsibility and, as the Government ultimately owns the Post Office, we call, once again, for Business Minister Margot James to order an investigation into the Post Office's future and what we consider is a catalogue of managerial incompetence."
The Post Office told customers that "almost all" of its network of 11,600 branches will be open despite the strike.
Kevin Gilliland, Post Office network and sales director, said: "We want to reassure people that almost all of our network will be open and operating normally.
"Action affects fewer than 300 of our branches and many of these will be open to maintain services for customers preparing for Christmas.
"It will also be business as usual in the rest of our network - over 97% of branches will be operating their usual Saturday service.
"Instead of trying to unnecessarily alarm customers during the festive season, we want the unions to join us in talks to discuss our strategy.
"Their members work very hard to give our customers the best possible service in the run-up to Christmas. This action can only cause concern to customers."
Sunderland CWU branch spokesman Steve Keeley said: "The union is dismayed by the company's plans for the wholesale privatisation of the crown Post Office network.
"The government has launched a public consultation on the future of the Post Office and, therefore, as part of today's action we held an event outside the Fawcett Street branch where our members and reps were joined by Sunderland Central MP Julie Elliott to ask the public to sign postcards in response to the consultation, calling for an end to the closure and franchise programme and for a new strategy for the Post Office based on growth.
"Hundreds of cards were signed by customers and the public were very much in support of the action taken with many of them stating that were disgusted with the way the current government is destroying this cherished public service.
"We are asking the Government for an immediate halt to the cuts and a fundamental change in direction from the Post Office to protect our members' jobs and the future of the Post Office network."