NEW YORK ( TheStreet) -- Shares of Citigroup ( C) plunged 16% on Monday as investors reacting to the U.S. debt downgrade dumped financials.

The stock's nose-dive was next only to that of Bank of America ( BAC), which sank 21% to $6.53 . Shares of JPMorgan ( JPM) and Wells Fargo ( WFC) shed about 9% each.

Rating agency Standard and Poor's said late in the session that its downgrade of U.S. debt will not affect its rating of the four banks. "None of the banks we rate in the U.S. has an issuer credit rating higher than the U.S. sovereign rating. The sovereign downgrade does not alter the government support assumptions that we factor into our ratings on four banks," the company said in its release.

The rout on Monday left the stock trading at $28.16, less than half its book value of $60.34 per share and 42% lower than its tangible book value of $48.75 per share.

Shares have dived 38% since the stock underwent a reverse stock split in May, a move designed to elicit buying from institutional investors, many of whom have restrictions on trading in stocks that trade below $5.

Despite the lackluster stock performance, Citigroup has largely enjoyed positive ratings from analysts, given its significant presence in emerging markets and relatively lower exposure to the mortgage mess in the U.S.

Analysts did not alter their views on the stock following the debt downgrade. Barclays Capital analyst Jason Goldberg reiterated his outperform rating on the stock on Monday, while David Hilder at Susquehanna and David Konrad at KBW maintained their neutral and market perform ratings.

In its 10Q filing, the bank highlighted the risk from a U.S. downgrade but sought to reassure investors, highlighting its strong liquidity position with $241.1 billion in unencumbered liquid securities. It also noted its well-diversified liquidity base. As of June 30, 2011, foreign government securities, foreign government guaranteed securities and foreign reverse repos comprised approximately 33% of these unencumbered liquid securities.

The bank estimated that in the event of a severe downgrade scenario, involving a possible two-notch downgrade of its senior/long-term debt rating, accompanied by a one-notch downgrade of its commercial paper/short-term rating could result in an additional $1.5 billion funding requirement in the form of collateral and cash obligations.

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