Striking dock workers blocked thousands of tourists in the main Greek port of Piraeus on Wednesday amid a new wave of protests against austerity cuts aimed at cutting the country's debt mountain.
Communist-affiliated unionists stopped all ships leaving Piraeus despite a late-night court order declaring the strike illegal, the Greek coastguard said.
Seven ferries with thousands of passengers waiting to board were blockaded, a coastguard spokeswoman told AFP.
"Nothing is operating," she said. "A group of around 80 unionists are keeping ships from sailing."
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.
Railway workers are holding a series of two-hour work stoppages until Thursday, disrupting inter-city trains and services to Athens International Airport.
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 a debt of nearly 300 billion euros whilst mired in a deepening recession.