Three large post offices were closed and many more run with skeleton crews across Devon and Cornwall after workers walked out in a row over pay.
Branches in Exmouth, Redruth and Liskeard failed to open at all yesterday with many of the 14 Crown Offices offering only a limited service with managers manning the counters.
It is the 12th day of action in a dispute over closures, franchising, job losses and pay which has been ongoing since Easter. In a separate action today and tomorrow, union members will also be refusing to actively promote financial products at the offices.
The Communication Workers Union (CWU) said its members are "fiercely opposed to the company's plans to close and franchise offices, slash jobs and impose a pay freeze".
The Post Office said it could not hand out above-inflation pay deals at a time when the business was losing millions of pounds.
CWU South West area representative Tony Lorton said staff had been hit with a pay freeze until 2015. He added that there had been disruption across the region, saying that strike action had been taken "as a last resort.
The union's deputy general secretary Dave Ward, added: "The company's plans are to downgrade the network, reduce services to local communities and hit jobs in the network.
"There is no mandate for this course of action and customers across the country are appalled at the reckless attitude of the Post Office towards these public services."
The union claims the company made £94 million profit last year, paying significant bonuses totalling £15.4 million primarily to senior managers. It says cuts to front-line jobs and services are "simply not right" and threatened to continue action into the Christmas period unless management "see sense and negotiate a fair deal".
The Post Office said 87% of the 372 Crown branches remained open for business.
Kevin Gilliland, network and sales director, said refusing to promote financial services was "irresponsible".
"The CWU are calling for an above inflation pay agreement when last year our overall performance was a loss of £116m.
"They have refused their members a fair and affordable pay offer of cash payments totalling up to £3,400 by April 2015, and are failing to recognise the financial realities faced by the Crown network."