The government plans have provoked a series of nationwide strikes
Air traffic, urban transport and public services have ground to a halt in Greece, where hundreds of thousands of people have walked out on strike.
Thousands marched through Athens in protest at privatisation, caps on pay and reforms to the pension system.
A small group of protesters threw stones and clashed with riot police, who fired tear gas.
But there were no reports of injuries and for the most part rallies in the capital passed off peacefully.
Country 'at a halt'
Public offices across the country were forced to close, with hundreds of thousands of people thought to have joined the nationwide strike, called by two unions representing some 2.5 million people.
Nearly 200 domestic and international flights and many train services were cancelled, and ferries were forced to remain in port.
State hospitals ran on skeleton staffs, while schools, universities and post offices kept their doors closed.
Bank staff, lawyers, journalists and civil engineers also joined the strike.
Thousands of people take to the streets in Greece
"The country has effectively come to a halt," said union spokesman Efstathios Anestis.
"Participation is very high, in many sectors it exceeds 90% of the work force."
There was no confirmation of that estimate from the government, and with journalists also on strike, reliable news was sparse.
Two marches wound their way through Athens, displaying opposition to the 2009 draft budget, due to go before parliament this week.
Many of the protesters called for increased government support - demanding the minimum wage be doubled.
They decried a 28bn-euro (£22bn) government rescue package to banks hit by the international credit crisis - one banner read: "Not one euro to support the capitalists."
The unions are also opposing the government's plans to privatise the loss-making Olympic Airlines.
The conservative government of Prime Minister Costas Karamanlis wants to press ahead with its privatisation plans and the overhaul of the country's debt-ridden pension system.
The plans include eliminating most early retirement schemes, merging pension funds and capping auxiliary pensions.
Mr Karamanlis has promised to shield consumers from price rises and compensate workers from privatised firms.
However, the plans have provoked a series of nationwide strikes and demonstrations.