The Post Office is to close and franchise a further 37 crown offices, according to the Communication Workers Union.
They said around 300 staff and 127 financial specialist roles will be lost.
The Post Office said the majority of its branches were open despite the strike.
Three hundred branches are striking, but the Post Office said around 250 of these were open.
The rest of the Post Office's network of 11,600 branches is not involved in the dispute.
Post Office group network and sales director Kevin Gilliland said: "We want to reassure customers that nearly all of our network is unaffected by the strike action."
Strikes by rail and Post Office workers are "solidly supported" according to unions.
Southern Railway RMT members have begun a 48-hour stoppage in a long-running dispute over the role of conductors.
Members of the Communication Workers Union will strike for five days this week, including Christmas Eve, in protest at job losses, the closure of a final salary pension scheme and the franchising of Crown Post Offices.
The union said there was strong backing for its campaign from the public.
General secretary Dave Ward accused the management of rejecting a peace offer and warned post offices will cease to exist on the high street if the planned closure programme continues.
Post Office group network and sales director Kevin Gilliland said: "It will be business as usual in almost all of our network, with over 50,000 Post Office people on hand to support customers as they make their preparations for Christmas."
A number of strikes will be launched by thousands of workers across the rail, post and airline industries in the lead up to Christmas.Read the full story ›
The Post Office said it was 'business as usual' but unions said Saturday's industrial action was getting solid support.Read the full story ›
Workers and managers are to stage another strike on Saturday amid a long-running dispute over jobs, pensions and closures.Read the full story ›
Certain types of broadband routers used by both the Post Office and TalkTalk were targeted in the attack - damaging internet connections.Read the full story ›
Workers will walk out for the second time in two months.Read the full story ›
Union members voted overwhelmingly for industrial action, at 83.2%, amid fears that the service is on a "path to extinction".Read the full story ›
Holidaymakers have been rushing to change up their travel money ahead of the EU vote.
Post Office Travel Money, which accounts for one in four of all UK foreign exchange transactions, said currency sales overall have surged by 74% year-on-year since the weekend.
It said on Tuesday that Post Office branch sales were up by 48.8% on the same period a year ago, while online purchases were up by 381%.
The value of the pound has see-sawed amid the uncertainty over whether the UK will decide to remain in the EU or vote for a Brexit.
There have been warnings that a Brexit vote could trigger further falls in the pound - and some holidaymakers may be hedging their bets by changing their money ahead of the vote.
International money transfer specialist HiFX reported the number of holidaymakers buying euros and US dollars has increased by 46% since the weekend.