Greece has set up a crisis unit for tourism after nearly 20,000 hotel booking cancellations in Athens alone after protests against government austerity measures, an official said on Thursday.

"Following mass cancellations of reservations, a crisis committee led by the Greek Tourism Organisation has been set up," government spokesman George Petalotis told reporters.

He estimated the cost in the "tens of millions of euros."

Tourism is one of the Greek economy's main drivers, generating 17 percent of gross domestic product.

But at a time when the debt-hit country needs visitors more than ever, Greece has been in the media spotlight for recurring anti-austerity protests, often violent.

"The incidents that stigmatised Greece internationally were decisive in creating a climate of uncertainty and fear for visitors who had made bookings and who now no longer want to come to Greece for obvious reasons," Petalotis said.

"There are thousands of cancellations because of the incidents of recent days and the economic shortfall created every week runs in the tens of millions of euros," he said.

According to the AFP, the capital's hoteliers union EXA said 68 of its members in the greater Athens region had reported nearly 20,000 night cancellations.

Three people died last week in an Athens bank firebombed during a demonstration and hundreds of tourists have faced delays at the main port of Piraeus due to successive general strikes.

A new general strike called by unions for May 20 will be joined by sailors, their federation said on Thursday.

"We have decided to join a 24-hour general strike on May 20 with a nationwide strike among all crews," the Panhellenic Seamen's Federation said in a statement.

The federation announced a second 24-hour strike for May 31 likely to disrupt for a second time the departure of a Maltese-flagged cruise ship which was prevented for hours from leaving Piraeus last month as well.