Demonstrators called on the socialist government of Michelle Bachelet to scrap the the system, which is managed by private funds.
Critics say the system benefits the rich but leaves poorer Chileans with a pension below the minimum wage. The system was introduced in 1981 under General Augusto Pinochet's rule. Protest organisers said a total of more than 2 million people had joined marches in most Chilean cities, and that 800,000 people took part in Santiago alone.
Chilean police put the number of demonstrators in the capital at 50,000. Chile's private pension system, known as AFP (Pension Fund Administrators), was once praised by pro-market politicians and economists across the world. But critics say it benefits the administrators and the wealthy, but leaves poorer Chileans with a final pension of less than $400 (£320) a month.
In her first government, Ms Bachelet reformed AFP and reduced the commission private management companies are allowed to charge.
