Riot police made heavy use of tear gas and stun grenades to disperse youths throwing stones and petrol bombs as thousands of demonstrators marched through central Athens to protest the Greek government's harsh austerity measures.

At least 25 civilians were injured Wednesday, including a man who suffered life-threatening head wounds and was hospitalized in critical condition, hospital officials said. Two police officers were also injured during the demonstration, officials said.

The police quickly announced they were launching an investigation into the man's injury and all other reports of casualties in the demonstration.

The clashes came during a 24-hour general strike that brought most public services to a halt, idled all trains and island ferries, grounded flights for four hours and disrupted public transport.

Police said at least 20,000 people marched in two separate demonstrations in the capital, and another 8,000 protested in the northern city of Thessaloniki.

The unions have joined many economic experts in questioning the effectiveness of more austerity at a time when the economy badly needs growth — or possibly a debt restructuring — to emerge from its debt hole.

The country is expected to need more financial help, beyond the $158 billion US bailout that saved it from bankruptcy last year. But European officials, who have ruled out the possibility of Greece reneging on debt accords, say any new help would require more reforms.

After a year of belt-tightening, some experts wonder how much more Greeks can realistically take and whether Europe should not reconsider its long-term crisis strategy.

This month, the socialist government is planning to pass further cutbacks aimed at saving an estimated $33 billion through 2015.

Twenty-five people were treated at two hospitals for various injuries or breathing problems, hospital officials said.

The most seriously wounded man was a 31-year-old man who suffered a very serious head injury. Doctors managed to stabilize him in surgery, but "his injuries remain life-threatening," a hospital official involved in his care said. The official spoke on condition of anonymity in line with hospital policy.