Greek workers banged drums, threw stones and walked off the job Wednesday in the widest strike yet against the government's austerity plan aimed at fixing the country's debt crisis.

The unrest flared despite a looming deadline for demonstrating tough cuts demanded by the European Union to calm the crisis and keep it from spreading to other countries with troubled finances such as Portugal, Spain and Italy.

Strikes grounded flights, idled cargo ships and ferries, and left commuters in Athens without most public transportation. State-run schools, tax offices and municipalities all shut down and public hospitals limped by using emergency staff.

In the capital, some 50,000 people marched through central Athens to protest spending cuts already imposed. The march itself was peaceful, with scuffles taking place after it ended, and comes after public opinion polls suggest many Greeks actually recognize the necessity of painful measures.

But Wednesday was the day for the unions to push back.

"We're all here for the same reason: the measures the government is taking. They have to listen to us," said musician Dimitris Petridis, who marched banging a snare drum with colleagues to a funereal rhythm.

"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," he added.

As the peaceful march ended, riot police clashed and fired tear gas at scores of demonstrators in the latest sign of unrest in recession-hit European countries. Groups of youths vandalized banks and storefronts, hurling rocks, red paint and plastic bottles near parliament. Three people were arrested.

Windows were also smashed at the Finance Ministry's General Accounting Office, which has been accused by the European Union of faking statistics for years to hide Greece's dire situation.

Greece is considering tougher austerity measures to ward off a financial crisis that has undermined the euro currency used by 16 European nations. Its troubles have raised fears that financial market contagion will spread to other weak eurozone economies such as Portugal, Spain and Italy.

The pressure on the Greek government to deliver on its promise to rein in the country's borrowing levels ratcheted up further Wednesday with the news that Standard & Poor's, one of the three leading credit ratings agencies, could downgrade its rating on the country within a month.

The EU has issued a vague promise to support Greece, which has some 53 billion euros ($72 billion) in debt coming due this year, but Prime Minister George Papandreou's new Socialist government has pressed for more specific guarantees to shore up market confidence.

Greece has already imposed broad spending cuts but says it is under pressure from the EU to cut salaries in the civil service. Unions say cutting Greeks' so-called 14th salary — part of annual pay held back as a holiday bonus — for public workers would be taken as "an act of war."

"If all these measures are enforced, unemployment will skyrocket. Our country will enter a massive recession and unemployment will reach a Europe-wide record," union spokesman Stathis Anestis said. "This will be tragic because it will provoke social (unrest) and clashes."

Officials from the EU and International Monetary Fund are in Athens to inspect public finances, ahead of a March 16 deadline to show signs of fiscal improvement or face imposed additional austerity measures.

Greece has promised the EU it will reduce the bloated budget deficit from 12.7 percent of gross domestic product to 8.7 percent this year. The country's woes have caused the euro to sink against the dollar and hiked the country's borrowing costs.

Greek unemployment hit a five-year high of 10.6 percent in November 2009, up from 9.8 percent in October.

S&P said it was maintaining its current rating on Greece's long-term debt of BBB plus but was monitoring the situation closely.

"In our view, a further downgrade of one to two notches is possible within a month," said Standard & Poor's credit analyst Marko Mrsnik.

A two-notch downgrade would take S&P's rating on Greece to BBB negative — the lowest level accepted by the European Central Bank as collateral for loans. Moody's current rating on Greece is A2, although that may be lowered soon.

Shares on the Athens Stock Exchange closed up 0.21 percent.

Elsewhere in Europe, other workers fearing job threats also took action this week.

In France, a strike by air traffic controllers disrupted flights for a second day Wednesday. In Spain, tens of thousands of demonstrators rallied Tuesday to protest a government proposal to raise the retirement age by two years to 67.

In Germany, over 4,000 Lufthansa pilots held a strike Monday to demand greater job security, causing travel chaos across Europe as over 800 flights were canceled.