The Greek government's reforms include tax hikes, public wage cuts and an increase of the retirement age.
The public spending cuts are planned in return for a $140bn bailout from the European Union (EU) and International Monetary Fund (IMF).
The aim is to cut Greece's public deficit to less than three per cent of GDP by 2014. It currently stands at 13.6 per cent.
Unions representing 2.5 million workers, half of the country's workforce, are particularly angry about a pension bill unveiled last week to raise the retirement age and curtail early retirement.
"We are here to send a strong message to the government, the Brussels directorate, the IMF and all those pencil-pushers who are targeting the social, labour, pension and economic rights of employees," Stathis Anestis, a leading member of the General
Confederation of Workers (GSEE), said in a speech to protesters.
Jacky Rowland, Al Jazeera's correspondent in Athens, said: "People are unhappy, they are dissatisfied with the austerity measures which the government is planning to introduce to meet the conditions of the EU and IMF.
"Sometimes the demonstrators are joined by other fringe groups, for example the anarchists, and when there is trouble it tends to be more sparked off by members of these kinds of organisations rather than the actual protesters from the various trade unions."
A 50,000-strong anti-austerity march on May 5 turned violent when protesters lobbed petrol bombs at a bank in Athens killing three employees, including a pregnant woman.
It was the worst violence to hit Greece since riots paralysed Athens for weeks in December 2008.
"The government has presented a draft bill which will lead to the collapse of the social security system and hurt young generations mostly. This bill must be withdrawn," Ilias Vrettakos, the deputy president of the public sector union ADEDY, said.
The protests, often marred by violence, have affected the key tourism sector [Reuters]
Opinion polls show most Greeks agree reforms are necessary to stem the country's debt crisis but they are angry because they believe the burden is being unfairly shouldered by the poor while the rich evade taxes.
The repeated strikes and protests, often marred by violence, have affected the key tourism sector, with industry bodies saying thousands of cancellations in Athens followed the May 5 violence.
George Papandreou, the Greek prime minister, told Al Jazeera he "understands" the public outrage.
"The fact that we have taken these emergency measures has shown our determination and our will, but I would say they do hurt," Papandreou said.
"I very much understand the protests but at the same time I think people are determined because of the pain, to say 'never again'.
"Let's really make these changes. Let's make Greece a viable economy and a competitive economy."
Air traffic controllers, members of ADEDY, said they would not strike on Thursday because they did not want to hurt tourism as Greece struggles with recession. But other unions walkouts are expected to affect domestic flights.
Labour unions have warned of more strikes if the government leaves the draft pension reform bill unchanged before it is voted in parliament early June.
Political analysts say the government could then face a short period of relative calm as Greeks flee the capital for the summer, but come autumn, people could demand to see their sacrifices are paying off or take to the streets.
Greece fully repaid a 10-year, $10.55bn bond on Wednesday, overcoming a major refinancing hurdle thanks to EU and IMF aid.