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Police have clashed with demonstrators as thousands marched through the Greek capital, Athens, in protest at the government's austerity measures.

At least 17 people were injured, one critically, as police used tear gas and stun grenades to disperse youths hurling stones and petrol bombs.

Police said there were several arrests and that two officers were hurt.

The march was part of a 24-hour strike that has brought public services to a halt and paralysed transport networks.

At least 20,000 people marched through the capital, police said, while another 8,000 protested in the northern city of Thessaloniki.

Those injured during the clashes in Athens were treated at two hospitals.

The most seriously hurt was Ioannis Kafkas, 31, who suffered a life-threatening head injury. Doctors managed to stabilise him in surgery but he remained in a critical condition, one official told the Associated Press.

Later, protesters gathered at the hospital where Mr Kafkas was being treated and attacked a policeman who arrived to investigate his injuries, a police spokesman said. Two other officers were slightly injured.

The strike was called by unions after the Greek government proposed a new austerity package to try to reduce the budget deficit and ease the country's crippling debt crisis.

Unions say the policies are making Greece's problems worse.

The violent clashes happened near to where Finance Minister George Papaconstantinou was meeting senior EU and International Monetary Fund (IMF) inspectors.

Many Greeks are angry over job losses, tax rises and pension and wage cuts, enforced by the government to meet the terms of an international bail-out.

But there is concern that the bail-out has not worked.

Protester Vangelis Papadoyiannis, a 46-year-old IT worker, told AFP news agency: "They want to suppress social rights acquired in past decades and take us back to the Middle Ages to save banks and bankers.

"In my company there were 100 layoffs just in January, our salaries were cut by 15% and there is more to come."

Litsa Papadaki, 60, a housewife also protesting in Athens said: "Enough is enough! They are killing us and our children."

Wednesday's one-day strike was called by the private sector union GSEE and the public sector union ADEDY.

A four-hour strike by air traffic controllers badly disrupted flights in a country heavily dependent on tourism.

The ´óÏó´«Ã½'s Nigel Cassidy in Athens noted that a large number of private sector workers had joined the strike.

"We strongly protest against the unfair and harsh policies that have pushed up unemployment, widen false employment and trample on worker rights," said the GSEE.

Unions said hospitals would be operating with skeleton staff only, schools would be closed, and all train and ferry services were being suspended.

Some banks were shut, while others were open but kept their shutters half-rolled down, fearing violence that might erupt during the day.

Last May, three bank workers died in a petrol bomb attack during anti-austerity demonstrations.

Despite its spending cuts, the government is failing to close its budget deficit as quickly as hoped - partly because the fiscal restructuring programme has compounded a recession, while unemployment has reached about 15%.

That has sapped market confidence that Greece will be able to avoid defaulting on its debts.

There is speculation that the government may need a further international loan, so that it does not need to raise capital on the markets, where it would face punishingly high interest rates.

The European Union's Economic Affairs Commissioner Olli Rehn said on Tuesday that talk of a new rescue package was "premature".

An EU mission is in Athens this week to review Greece's progress on meeting the terms of the 110bn euros ($158bn; £97bn) joint EU-IMF emergency bail-out agreed last year.

It has another two years to run and the mission's assessment will be key to deciding whether Athens will be offered better terms on the loans.