Greek police estimated that hundreds of thousands of people took to the streets Thursday in nationwide strikes against pension reforms and tax increases demanded by the country's creditors.

Greek media described the action as the largest in years, with one radio station reporting that it was "easier to say who is not on strike, than who is."

"Hands off our pensions," protesters chanted.

More than 100,000 demonstrators marched through the centre of Athens, where additional police were deployed as the government feared riots. There were afternoon clashes between police and a group of about 500 masked demonstrators described as anarchists.

One banner read: "The labour minister should support his family with 360 euros a month."

Protesters rallied in other cities including Thessaloniki, Patras, Heraklion and Volos.

The massive general strike was called by both private and public sector unions, and is being joined by Greeks from a wide range of occupations.

Public transport ground to a halt, while trains were cancelled and ferries stayed put in harbours, cutting off islands from the Greek mainland. Domestic flights were affected.

Ministries and schools were closed. Most pharmacies and shops lowered their shutters.

Lawyers didn't show up at courts while farmers continued to block key roads. Doctors in state hospitals were only treating emergency cases.

Theodoros Ioannides, who works in the pharmaceutical industry, told dpa that the protesters were a "mixed" group of "people from all sections of society."

Greece has been required to implement tough economic reforms in return for international bailouts that have prevented the country from going bankrupt.

One of the creditors' biggest demands under the current rescue package has been reform of the pension system.

In Washington, International Monetary Fund chief Christine Lagarde defended the crisis lender's demand for reforms in Greece.

"I have always said that the Greek programme has to walk on two legs: one is significant reforms and one is debt relief," she said.

"If the pension [system] cannot be as significantly and substantially reformed as needed, we could need more debt relief on the other side. Equally, no [amount of debt relief] will make the pension system sustainable."

The Greek pension system is costing 10 per cent of GDP, which Lagarde called "not sustainable" compared to a European average of 2.5 per cent. The crisis requires short-term measures that will make the pension system "sustainable in the long term," she said.

"I really don't like it when we are portrayed as the draconian, rigorous, terrible IMF," Lagarde said.

After several years already under international bailout conditions, Greeks "have already made a lot of sacrifices," she acknowledged.

But the pension system needs to be reformed, she said.

"The tax collection needs to be improved so that revenue comes in and evasion is stopped. And the debt relief by the other Europeans must accompany that process."

Prime Minister Alexis Tsipras' government is under pressure to implement the pension reforms, which include cuts averaging 15 per cent for new pensions.

He has insisted that the reform is absolutely necessary, saying that pension funds will otherwise run dry in five years. But Tsipras and his leftist Syriza party might face a showdown in parliament, where many lawmakers will heed the call of their constituents to say no to the reforms.

The government has a slim majority of three deputies, and any deviations will plunge the country into a fresh political crisis.

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