Greek workers went on strike on Thursday ahead of a parliamentary vote on a sweeping pension reform, in a fresh test of the Socialist government's resolve to tackle its debt crisis through austerity measures.

Flights to and from Greece were expected to be disrupted throughout the day, ferries kept in port, public transport frozen and public offices shut down, as unions stage their sixth 24-hour strike against austerity this year.

Thousands of people were expected to march to parliament around midday, where lawmakers were expected to debate a reform bill that curbs early pensions, raises the retirement age to 65 for all and cuts pension benefits.

"It's horrible, we paid all our contributions, we paid our taxes, and we are not going to get anything," said one of the protesters, 54-year-old private sector worker Vaso Spoulou, who had just filed a request for early retirement for fear that the reform would cut her pension. "They are destroying everything."

The reform is a key condition for an austerity deal agreed with the European Union and the IMF in return for a 110-billion-euro ($138.6 billion) aid package to pull the country out of a severe debt crisis.

Lawmakers agreed in principle on the reform in a preliminary vote late on Wednesday. It was a first sign that the bill was also likely to pass the reform during a final vote on Thursday, despite grumblings in the ruling party ranks.

Analysts see it as a major test of the government's ability to push through structural reforms. Many Greeks are angry with the prospect of having to work longer for a smaller pension, particularly women who can now retire as early as 55.

Prime Minister George Papandreou told lawmakers that without this reform the country's economy "would fall apart" under the weight of increasing pension costs.

Unions say plans to increase the early retirement age to 60, raise the effective average retirement age to 63.5 years by 2015 from 61.4 now, scrap bonuses, merge pension funds and cut the number of "arduous" professions mean the end of social security.

Athens, which was quiet ahead of the protest rallies, was covered in union posters saying: "All together so that the collapse of the social security system does not pass."

Parliament workers said they would also strike, in what is their first walkout in 25 years. It was not immediately clear how this would affect the voting process.

Turnout at rallies on strike days has fallen -- about 12,000 people marched in Athens in the last joint stoppage on June 29, down from 50,000 in the biggest rally on May 5, when three people died in the fire-bombing of an Athens bank.

But some unions which did not participate in some previous strikes, such as air traffic controllers, said they would join Thursday's walkout.

"This is Black Thursday. As of tomorrow nothing will be the same. Greece is changing and sinking into a period of Middle Ages for labor," center-right daily Eleftheros Typos wrote.

An airport spokeswoman said 82 flights had been canceled and 109 rescheduled. The Acropolis, often closed during strikes, was open on Thursday.

Greece's private and public sector unions represent about 2.5 million workers, or half the country's workforce. Their repeated strikes and protests have hit tourism, a vital sector for the Mediterranean country.