Announcing the move on television Sunday, U.S. Treasury Secretary Henry Paulson said it would bring stability and confidence to the markets.
"Fannie Mae and Freddie Mac are so large and so interwoven in our financial system that a failure of either of them would cause great turmoil in our financial markets here at home and around the globe," he said.
In a statement released Sunday afternoon, President George Bush said state control of the two companies was "critical" but stressed it would be temporary.
"Americans should be confident that the actions taken today will strengthen our ability to weather the housing correction and are critical to returning the economy to stronger sustained growth," he said.
Fannie Mae and Freddie Mac, which own or guarantee around $5 trillion in U.S. mortgage debt, have lost more than $12 billion in the past year.
The need for government action was supported by both major candidates in November's presidential election.
"Given the substantial role that Fannie Mae and Freddie Mac play in our housing system, I believe that some form of intervention is necessary to prevent a larger and deeper crisis throughout our entire economy," Democratic candidate Senator Barack Obama said in a statement, adding that the root of the crisis needed to be addressed.
Republican candidate Senator John McCain said the move was a step in the right direction, according to his economic advisor, Nancy Pfotenhauer.
"Fannie and Freddie are an example of crony capitalism," she said on CNN. "We need to protect the taxpayers and need to never allow this to happen again."
The Russian Central Bank holds tens of billions of dollars' worth of bonds in the mortgage firms, which yielded over $1 billion in the first half of 2008. At the start of the year, it held around $100 billion worth of the bonds, which are guaranteed by the U.S. government, but has cut that by more than half since the firms' troubles became clear.
