Members of the Communication Workers Union and Unite walked out in protest at the closure of offices, job cuts and changes to the pension scheme.
The Post Office said almost 99% of its 11,600 branches remained open despite the action, although 118 of the 305 Crown offices were closed.
Dave Ward, general secretary of the CWU, joined picket lines, saying the Post Office was at "crisis point", and urging the Government to stop the "cycle of closures, job losses and attacks on workers' terms and conditions".
The unions said the strike was solidly supported and 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 can reassure customers that ... nearly 99% of Post Office branches are open for business as usual and there to serve our customers.
"We apologise to any customers who have been inconvenienced by the disruption to service in a very small number of branches."
On pensions, Mr Gilliland said: "It is crucial that we safeguard the benefits that members of our defined benefit (DB) plan have already built up.
"The business's financial position is improving but we remain loss-making. The fund's surplus is currently being used to help subsidise the cost of the plan and, based on the advice of our actuary, it will run out in 2017.
"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."