Employees on ferry services in Greece went on strike on Wednesday, which is International Workers' Day or Labour Day, but also the middle of the Orthodox Easter holiday period.
The umbrella union PNO was demanding higher wages and better health care for its members.
In the major cities of Athens and Thessaloniki, public transport workers also went on strike.
The conservative government and Prime Minister Kyriakos Mitsotakis had previously suggested that the unions postpone the strike to May 7 because many Greeks travel to their home villages and the islands at Easter and the tourist season has also begun.
"May 1st is Labour Day, not May 7," the unions said. They called on their members to demonstrate nationwide for better working conditions.
Large marches are expected in Athens.