Their protest echoes grievances in other parts of Europe, including France, Belgium, the Netherlands, Poland and Italy, where farmers have staged similar demonstrations.

Greek farmers' unions have been in negotiations with Prime Minister Kyriakos Mitsotakis' conservative government for weeks, but say the measures announced so far don't go far enough to meet their concerns.

On Monday night, farmers lined their tractors and pickups along highways as they gathered at a meeting point in central Greece where they spent the night before heading to Athens.

"There are many problems, most of all the fuel and the energy costs," said one of the protesting farmers, Christos, in the central Greek town of Kastro.

"Last year was catastrophic for farmers, we did not produce grapes, we did not produce olive oil, we produced a bit of cotton but it was bought for nothing."

On Tuesday morning, dozens of farmers arrived by boat at the port of Piraeus from the southern island of Crete. More farmers were expected to arrive by bus from other areas across Greece.

The rally, scheduled for 1530 GMT, was aimed at piling pressure on the government, which has already offered discounts on power bills and a one-year extension of a tax rebate for agricultural diesel to the end of 2024.

The government reiterated on Monday that it is willing to discuss a more permanent tax rebate scheme in the future, but it had no fiscal room for any further concessions this year. Greece has been recovering from a decade-long financial crisis.

"We have nothing more to give," Mitsotakis said during an interview with Greek Star TV on Monday evening.

"I think farmers acknowledge this and know very well that the government has probably exceeded even their expectations, especially on the power bills issue."

He said the rally was expected to be largely symbolic, but farmers appeared determined to push for more concessions.

"We'll see if there is anything else at the bottom of the barrel," said protesting farmer Dimitris Tsarouchas.

(Reporting by George Moutafis and Renee Maltezou; Editing by Sharon Singleton and Christina Fincher)