Citigroup shares dropped 7.6 percent on Thursday morning, reaching their lowest level in over four years, after an analyst said the largest U.S. bank may have to raise $30 billion of capital.

Pressure on the company's capital, and shares, will likely further intensify calls for Citi CEO Chuck Prince's ouster, portfolio managers said.

Meredith Whitney, analyst at CIBC World Markets, said in a note to investors that with Citigroup's current capital levels, Citi might have to raise $30 billion through moves such as selling assets or cutting dividends.

Whitney downgraded Citi to sector underperformer from sector performer.

I think the market is just fed up with Prince, said Lee Norton, an analyst covering financials at JS Asset Management, which owns Citi shares.

I've been in Prince's corner for some time now, but even I'm ready for a change, Norton added.

Citi shares dropped $3.15 to $38.21 on Thursday morning. The decline in percentage terms was the biggest one-day drop since 2002.

(Reporting by Dan Wilchins)