The scheme was launched in 1981, during the military government of General Augusto Pinochet.
Protesters say some 10 million people who joined have now been left with very low retirement incomes - less than minimum wage in many cases.
They are calling for President Michelle Bachelet to scrap the scheme.
The Chilean pension fund system has been praised by pro-market politicians and economists across the world.
But critics say it pays many contributors less than the minimum wage, far less than the 70% of final salary originally intended.
President Bachelet, who is left wing, announced a number of reforms earlier this month to try to boost the income of pensioners.
She proposed an increase in employer's contributions and a reduction in commissions paid to the fund managers.
But protesters want the Pension Fund Administrators (or AFP) scheme to be scrapped altogether.
Leaders of the No More AFP movement have called a nationwide strike on 4 November.
Hundreds of thousands of people across Chile have taken part in protests against the country's controversial privatised pension plan.