Post Office management has claimed the strike is causing only a modest disruption in services, but Communication Workers Union leaders said the impact of the strike was more acute, The BBC reported Saturday.
"We know that the larger offices are being kept open with only the manager present as they are not members of our union, but our members are on strike," a CWU spokeswoman said.
Union leaders said workers had not received a pay raise in two years. The union is also protesting a plan to close or franchise out 70 of the so-called crown Post Office branches.
"We cannot allow the Post Office to walk all over us, we do need to stand up for ourselves so this could be the first of many strikes," said Andy Furey, the union's assistant secretary. "Hopefully it won't be. Hopefully the Post Office will come to the bargaining table."
The strike involved staff at 370 crown branches.
The Post Office manages 11,500 branch locations in Britain, 10,000 of which are franchised, The Guardian reported.