U.S. stocks fell on Monday amid continuing uncertainty over the health of the financial markets, brought into greater doubt with Citigroup's huge write-down on losses related to mortgage-backed securities.

Citigroup said Monday it would post as much as $11 billion in additional write-downs losses, without reassuring investors that more write-downs are on the way. Citi's Chairman and Chief executive, Charles Prince announced that he would resign as a result of the losses.

The blue chip Dow Jones Industrial average fell 51.7 points, or 0.4 percent to 13,543.4. The technology stock heavy Nasdaq Composite Index fell 15.2 points, or 0.5 percent, to 2,795.18. The broader Standard & Poor's 500 Index dropped 7.48 points, or 0.5 percent, to 1,502.17.

Shares of Citigroup fell $1.83, or 4.9 percent, to $35.90. The company's stock price has fallen 32 percent since the start of the year.

Other financial stocks also fell including Morgan Stanley, which dropped $3.31, or 5.62 percent to $55.59 after CNBC, citing unnamed sources, reported that the firm would post a write-down of $3 billion. Shares of Goldman Sachs Group slipped $11.21, or 4.99 percent, to $218.39. Last Friday, the company strongly denied truth to rumors that it was set to announce billions of dollars in write-downs. Bear Stearns fell $2.25, or 2.20 percent, to $99.91. Merrill Lynch continued its slide, falling, $1.40, or 2.44 percent, to $55.88.

Meanwhile, shares of Google rose $14.40, or 2.02 percent, to $725.65 as it announced its entry into the mobile phone market by offering a software system to a broad array of cell phone industry firms to make Internet use, and ad sales, easier.

Home improvement chains Home Depot and Lowe's retreated after Deutsche bank downgraded the stock on to lower profit expectations due to the housing downturn. Home Depot shares fell $0.60, or 1.97 percent to $29.80. Lowe's fell $0.87, or 3.35 percent, to $25.13.

Citigroup Investment Morgan Stanley U.S.