Thousands of tourists were blocked at the main Greek port of Piraeus on Wednesday and access to Athens airport was disrupted in the latest in a wave of strikes against sharp austerity cuts.
Communist-affiliated unionists stopped all ships leaving Piraeus despite a late-night court order declaring the strike illegal, the Greek coastguard said.
Eight ferries with thousands of passengers waiting to board were blockaded so far by the one-day mobilisation, the coastguard said.
"Travellers and operators alike are going through a great ordeal," said George Telonis, vice president of the Greek association of travel agencies (HATTA).
"This constitutes defamation for the country," he told AFP.
The Communist-affiliated Pame union, which ignored calls from the sailors' union to lift the blockade and is holding other protests in main Greek cities, said the action will last until early Thursday.
A few hundred Pame protesters marched through the Athens centre Wednesday, chanting slogans against the European Union and the International Monetary Fund who are involved in a Greek debt bailout but are also seen here to have imposed unprecedented budget belt-tightening on the country.
"What is paramount here is to prevent my child and yours from working as a slave, not going for a dive on Mykonos," Pame unionist Yiannis Manousogiannakis told Flash Radio, referring to the popular Cycladic holiday island that draws many thousands of travellers every summer.
The ferries cannot depart for safety reasons as ship engineers are among the strikers, coastguard chief Athanassios Bousios told the station.
He added that the coastguard would not attempt to break the blockade to keep violence from erupting in the harbour.
"Operationally this is a risky mixture, having unionists and passengers on the scene," Bousios said. "We do not want to make things worse."
Rail access to Athens airport was also disrupted as railway workers were holding a series of two-hour work stoppages until Thursday.
A 24-hour strike by TV journalists against the budget cuts forced the state channel to interrupt its regular programme. The fifth general strike since the start of the year has been called for June 29.
Strikes and street protests have hit Greece in recent months over draconian pay and pension cuts, bringing havoc to the vital tourism industry. Seeking to stem a wave of cancellations, the government this week promised to reimburse stranded travellers.
"The Greek state will ensure that (travellers') stay will be covered, so that they will know that Greece as a destination will cost them no more than what they had originally planned," Culture Minister Pavlos Geroulanos who has overall responsibility for tourism said on Monday.
Tens of millions of euros have already been lost from cancellations according to government estimates.
Greece adopted the austerity cuts to secure a 110-billion-euro (135-billion-dollar) bailout loan from the European Union and the International Monetary Fund and save itself from default.
It is struggling to reduce debt of nearly 300 billion euros whilst mired in a deepening recession.