Around 600 staff in the 40 Crown offices
and the 33 counters currently directly managed will be affected.
The union said a series of recent announcements would lead to compulsory redundancies at Crown offices
and other areas of the Post office.
The Post Office argues that moving crown offices
- the larger branches usually on high streets - to other sites such as retail outlets maintains and improves services.
The union said staff in the country's 372 Crown offices
have not had a pay rise since April 2011, and faced an uncertain future because of plans to close or franchise more than 70 of the branches.
The 372 Crown offices
nationally - which handle 20% of all Post Office business - are directly run by Post Office Ltd rather than franchisees.
Picket lines were due to be mounted outside the Crown offices
today, with up to 4,000 union members nationally involved in the action.
The Post Office says the Crown offices
are losing PS40m a year and has accused the union of refusing to accept economic realities.
Andy Furey, CWU national official, said: "It's ironic that the Post Office has been trying to say offices remained open today when, by their own plans, they want to lose 76 Crown offices
. That's 20 per cent of the network and would hit communities badly.
A number of Crown offices
have already been franchised to firms such as WH Smith.
The Communication Workers Union is taking employment tribunal cases on behalf of 1,300 workers affected by the programme of franchising postal services from the Crown offices
- the larger branches which are usually based in high streets.
The union is protesting at plans to close down 70 Crown offices
- main branches on high streets - and transfer services to WH Smith stores.
Communication Workers Union deputy general secretary Dave Ward said: "Crown offices
provide an unrivalled service and are an integral part of communities up and down the country."