Greek dockers on Thursday said they would hold a 24-hour nationwide strike on October 22 to protest against the planned privatisation of the Piraeus and Thessaloniki port authorities.
"Despite its pledges, the government is attempting to hand over critical shipping, tourism, import and export activities to foreign monopolies," the OMYLE union said.
An early morning work stoppage will also be held on October 21.
The left-wing government of Prime Minister Alexis Tsipras in July signed on to a three-year EU bailout confirming a number of key privatisations.
Offers for a 51-percent stake in the Piraeus port authority (OLP), which manages the harbour's maritime services and part of its container traffic, are expected later this month.
Tsipras, who was re-elected last month, had earlier vehemently opposed the projects.
Shipping minister Thodoris Dritsas on Wednesday told parliament that the government was "obliged" to carry out the sale, followed by a similar process for the port of Thessaloniki.
"We are bound to this obligation," Dritsas said, but he added that the leases would take into account "the needs of workers, local society and municipalities."