Unions say around 2,000 postal workers in Bern, Zurich, Basel and other Swiss cities and towns staged a protest action on Friday.
They were demonstrating against a major cost-cutting programme and plans to scrap their collective labour contract.
A two-hour work stoppage also took place in Geneva, Lausanne, Biel, Lucerne, St Gallen and in canton Ticino.
The Communications Union claims changes planned by Swiss Post to the collective work contract will lead to increased hours, lower wages and the dismantling of a public service.
The company has said it wants to introduce a regional salary system and a 41-hour week.
The current collective contract runs out at the end of the year; negotiations between Swiss Post and the union are due to start in March.
Christian Levrat from the Communications Union told swissinfo that his side would not enter talks until it had received certain guarantees.
“[These are] job security, a minimum wage and no regional salary system,” said Levrat.
The union warned on Friday that there would be further nationwide action on April 21 unless management made concessions.
The Swiss Post Office says that drastic measures must be undertaken for it to remain competitive in a liberalised market.
Spokesman Oliver Flüeler told swissinfo: "We have always said that our employees enjoy far better working conditions than others in this sector. However, if we do not become more competitive, this will result in significant problems."
Earlier this year, the company announced a major restructuring plan affecting its mail, logistics and transport units.
It said it would close post offices and outsource work, potentially resulting in more job losses on top of the 3,000 announced last year.
Announcing the restructuring, Swiss Post director Ulrich Gygi said postal workers were overpaid - a claim that roused the ire of trade unions.
“The employees saw this comment as a provocation. We were also upset because Gygi had spoken out publicly without informing us first of his point of view,” stated Levrat.
Swiss Post remains one of Switzerland’s largest employers with 56,000 staff.
swissinfo with agencies
Swiss Post belongs to the government but has been autonomous since 1998.
During the last few years, its monopoly has been gradually eroded – since the beginning of 2004, private companies have been able to send parcels weighing less than 2kg.
Swiss Post deals with 17 million letters and 500,000 parcels daily in nearly 3,000 branches.
Annual turnover is SFr6.2 billion ($4.9 billion).
Postal workers across Switzerland protested against revising their collective work contract and restructure plans.
They downed tools for two hours in a number of Swiss towns on Friday.
Swiss Post wants to scrap a collective labour agreement for regional salaries and a longer working week.
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