At least 25 civilians were injured, 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 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 take and whether Europe should not reconsider its long-term crisis strategy.
Twenty-five people were treated at two hospitals for various injuries or breathing problems, hospital officials said. Doctors managed to stabilize a 31-year-old man who suffered a serious head injury, but "his injuries remain life-threatening," a hospital official involved in his care said.
Protesters gathered at the hospital and hit policeman who arrived to conduct an investigation, police spokesman Athanasios Kokalakis said. He said three policeman were slightly injured.
A local member of parliament, Panagiotis Lafazanis, who was briefed by doctors about the man's condition, said it was expected to remain critical for several days.
Angered by the man's wounding, about 300 people demonstrated in Thessaloniki against police brutality on Wednesday evening before dispersing peacefully. Another 100 people gathered outside the Athens hospital where the man was being treated. A media blackout due to the strike meant news about his condition was available mainly through online blogs.
Police said 30 suspected rioters were detained in Athens, with 11 of them arrested as dozens of black-clad anarchists smashed bus stops, set rubbish bins on fire and smashed a shop window.
The fighting divided the day's second march called by the two main labor unions -- which was otherwise peaceful -- into two. Choking clouds of tear gas sent demonstrators and tourists scurrying for cover past shops and banks that had their fronts shuttered in anticipation of trouble.
Previous protests have also been marred by violence. Three clerks died last May when their bank was torched by rioters.
"Every day that passes (the government) takes back what the working class has won through blood and struggles all these years," retiree John Pavlidis said.
Greece's crisis follows years of inept governance, widespread corruption and waste that created bloated budget deficits and a public debt considerably larger than annual economic output. For the time being, the country is shielded from insolvency by the 110 billion, 2010-2013 program from its European Union partners and International Monetary Fund.
Parliament is to vote on the new round of cutbacks later this month. The governing Socialists have also committed themselves to an ambitious -- but so far nebulous -- privatization program worth a total of 50 billion ($72 billion) over the next few years.
However, many promised reforms have not yet been implemented, and there is growing skepticism in Greece and abroad over the government's efficiency.
"Before we can talk about further aid, Greece has to make sure that all austerity and reform measures are duly implemented," said Michael Meister, a deputy caucus leader of German Chancellor Angela Merkel's conservative party.
"I would like to have a signal that this is finally happening," the lawmaker told Wednesday's German Rheinische Post daily.
EU and IMF officials are currently in Athens for talks on the austerity program -- on which the continued release of the bailout loans depends.
Greek unions say the protracted austerity, amid a two-year recession and unemployment at around 15 percent, is unfairly targeting the less well-off.