Greek farmers have clashed with riot police in Athens during protests against new austerity measures demanded by the European Union and the International Monetary Fund (IMF).

Some 10,000 farmers from across the country were joined by worker unionists in central Syntagma Square on Friday, waving Greek flags and led by 20 tractors loudly blaring their horns.

Al Jazeera's Neave Barker, reporting from Athens, said: "Many of these farmers have already taken part in up to two weeks of protest, blockading various highways across the country."

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Earlier in the day, farmers from the island of Crete clashed with police outside the agriculture ministry after arriving by ferries at the port of Piraeus.

They pelted the police with tomatoes and other items, broke windows in the ministry, and set fire to dustbins. Police, who blocked their route, responded with tear gas, and arrested four people.

"The first floor of the building sustained damage; it is fortunate that no staff were hurt," Agriculture Minister Evangelos Apostolou called on the farmers' leaders to contain "extreme" elements in their midst.

The junior interior minister for police, Nikos Toskas, said 10 police officers had been hurt, two of them requiring hospitalisation.

Painful reforms

Since mid-January, farmers have used their tractors to block dozens of highways.

They are angry about government plans to increase their social security contributions as part of a wider reform of the country's ailing pension system. They also reject plans to double their income tax by 2017 and scrap benefits, such as cheaper fuel.

"About 70-75 percent of our incomes will go on taxes and pensions. This is not affordable. It's the death of the farmer, especially the small and the middle-class of farmers," Konstantinos Massouris, a farmer, told Al Jazeera.

Greek Prime Minister Alexis Tsipras said the reform was "not optional, nor merely a contractual obligation of the country".

"It is absolutely necessary for the pension system itself to have a future," he said.

Greece's creditors, the EU and the IMF have insisted on the reforms as a condition for loans to the debt-ridden country.