Police have fired tear gas at demonstrators in central Athens as clashes broke out during a large protest march against government austerity measures, AP reports.

Police clashed with scores of violent protesters who hurled rocks and plastic bottles near parliament, but the violence remained fairly limited.

More than 30,000 protesters took part in the march Wednesday, as unions staged a general strike which grounded flights, shut schools and crippled public services in a show of strength against the government austerity measures.

The 24-hour walkout comes amid reports Greece is considering tougher austerity measures, including deeper salary cuts and drastic reforms of the civil service and pensions system, to pull the debt-ridden country out of financial crisis.

Chanting "Billions for the rich, but nothing for us," demonstrators marched to parliament, as riot police maintained a discreet presence.

Police pushed back a small group of marchers who pelted them with plastic water bottles, but the incident ended quickly.

Musician Dimitris Petridis, banging a snare drum along with colleagues to a funereal rhythm, said a spike in unemployment had hurt business.

"We're all here for the same reason: the measures the government is taking. They have to listen to us," Petridis said, while keeping up the drum beat. "The rise in joblessness has really hurt us. The daily wage for working at a nightclub, for many of us, is the same as it was 20 years ago."

Demonstrations, part of the first general strike held by unions since the center-left government's election in October, were held also in cities across Greece.

Wednesday's strike grounded all flights at Greek airports and left trains and ferries idle. Commuters in Athens were left without most forms of public transport. State-run schools, tax offices and municipalities were all closed, while public hospitals used emergency staff. Journalists also held a 24-hour strike.

The country's two largest umbrella labor groups, the private sector GSEE and public sector ADEDY, fiercely oppose a wave of belt-tightening measures announced over the past weeks to reduce the bloated budget deficit from 12.7 percent of gross domestic product to 8.7 percent this year.

"If all these measures are enforced, unemployment will skyrocket. Our country will enter a massive recession and unemployment will reach a Europe-wide record," GSEE spokesman Stathis Anestis said.

"This will be tragic because it will provoke social (unrest) and clashes."

Greek unemployment hit a five-year high of 10.6 percent in November 2009, up from 9.8 percent in October. The country's woes have affected confidence in the euro as a common currency, and hiked the country's borrowing costs.

The governing Socialists have frozen civil service wages and hiring while cutting bonuses, hiking consumer taxes and raising retirement ages.

Greek borrowing rates nevertheless remained high on Wednesday, reflecting market worries of a default. Spreads on government bonds over their German equivalent was at 330.1 basis points after Fitch ratings agency on Tuesday downgraded ratings for four Greek banks.

Shares on the Athens Stock Exchange were up 0.49 percent in early afternoon trading.

Greece is facing a March 16 deadline from the European Union to show signs of fiscal improvement and is under pressure to take additional measures. These could include a hike in the Value Added Tax, currently at 19 percent, and further civil service bonus cuts.

Greece's central bank governor George Provopoulos said in a speech Tuesday that Greece's crisis heightened a pressing need for major economic reforms.

"The crisis could present an opportunity to carry out necessary reforms -- and not just have a debate about them -- given that not implementing these reforms would have a great price."

Wednesday's strike will be a crucial test of support for the unions, with polls showing strong public support for the government's austerity plan.

A poll Sunday in the Ethnos newspaper showed some 57.6 percent of Greeks believe measures taken so far are "in the right direction," while 75.8 percent think unions should show restraint until the end of the crisis.