Protesters estimated in the hundreds of thousands heeded a call by Greece's biggest unions to strike, shutting down schools, government offices, courts, airports and other transportation to oppose plans to change the pension system.

The Greek General Confederation of Labor, or GSEE, the umbrella union for two million private-sector workers, said participation in the nationwide strike was at least 90 percent.

"This is a strong message of rage," Yiannis Panagopoulos, GSEE president, said.

Stathis Anestis, a spokesman for the union, estimated the number of people at the march in Athens, the capital, at 200,000. Police did not estimate the number of protesters in Athens but said 30,000 rallied in the northern city of Thessaloniki.

Greece's pension system, ranked among the most generous in Europe by the Organization for Economic Cooperation and Development, will face a crisis within a decade as the population ages, the central bank's governor, Nicholas Garganas, has said.

Shopkeepers rolled down their shutters and bystanders raced for cover as riot police fired tear gas in clashes along the Athens demonstration route. Youths threw rocks at police and set garbage cans and a parked car on fire.

More strikes are expected next week, with medical professionals such as pharmacists and dentists walking off the job Wednesday.

The pension funds face estimated future deficits of $165 billion to $550 billion, sums expected to affect the budget within a decade.

Giorgos Skiadotis, who handles union issues for Greece's main communist party, said, "Workers should not pay for the mistakes and mismanagement of past governments in handling the pension funds."

Prime Minister Costas Karamanlis, whose pro-business New Democracy party was reelected in September with a slimmer majority, has promised changes to pension rules in the first half of next year but pledged not to raise the retirement age - 65 for men and 60 for women - or contributions.

The government plans to reduce the number of pension funds - there are 155 separate funds - as well as tighten rules for early retirement and provide incentives for employees to continue working longer.

This article contains information from the Associated Press.