The private pension funds strongly criticised the measure as a "short-term" move and insisted that their assets remained healthy.
Local media cited official sources as saying that the funds had incurred losses of around 20 per cent, on account of the turbulence in global financial markets.
The 10 firms together administer around $30bn in retirement savings of 53 per cent of Argentinian workers, and take in about $4.6bn each year in new contributions.
Eight of the 10 are controlled by private banks. One is a co-operative and another is controlled by state-owned Banco Nacion, the country's most important bank.
The political opposition accused the government of confiscating the private pension funds to help service the national debt of some $150bn.
Luis Corsiglia, a stockbroker in Argentina, said: "The market was shattered by the drastic change to the rules of the game concerning the pensions. It amounts to a confiscation of private capital."
But as she presented the nationalisation plan, Kirchner said: "We are taking this decision in the international context in which the G8 countries and others are seeking ways to protect banks.
"We are protecting our retirees and workers."