Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland walked out. There will also be strikes in a handful of branches in Scotland today.
The union said the stoppage, the 12th round of industrial action since Easter, was well supported, warning that the dispute would not go away. It is opposed to plans to franchise or close 75 Crown offices.
The Post Office said almost nine out of 10 of the Crown offices were open despite the strike.