NEW YORK (CNN/Money) - Citigroup CEO Charles Prince told his employees that the company is taking the reported threat against the company's headquarters very seriously.

Prince and other senior executives spoke with employees through a telephone conference call and internal Webcast Monday, the day after Homeland Security Secretary Tom Ridge identified the company's New York headquarters as one of the possible targets of al Qaeda terrorists. Prince said that the company is stepping up security procedures, but that he and others are confident it will not disrupt company operations.

"In this new world in which we all live, as we go about our daily lives, we need to be extra vigilant," Prince said. "That's true for the company and true for each of us as individuals. I have no doubt we'll rise to the occasion."

Company executives said besides additional screening of individuals and packages, they are taking steps that are not as visible to deal with the threat.

"We've known for a while that terrorists want to disrupt global financial systems and the fact that financial institutions are targets is not a surprise to us," said President and Chief Operating Officer Bob Willumstad.

In addition to Citigroup New York headquarters, Ridge identified the New York Stock Exchange, the Newark, N.J., headquarters of Prudential Financial and the homes of the International Monetary Fund and the World Bank in Washington as possible targets.

CNNfn's Mary Snow reports outside the Citigroup headquarters in midtown Manhattan on increased security and how employees feel about it. Play video (Real or Windows Media)

NYSE CEO John Thain said Sunday that he had spoken with Ridge and New York Mayor Michael Bloomberg about the threat posed to the exchange.

"The NYSE security staff will continue to work with our law enforcement partners, and Homeland Security officials on a 24 hour per day, 7 day a week basis to ensure we maintain the highest levels of security," he said in a statement. "The exchange will continue to be open for business on our normal schedule."

Bloomberg rang the opening bell at the exchange as a show of support.

Prudential said its offices were open despite the threat.

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"Prudential is already coordinating with local, state and federal law enforcement officials and has taken additional security precautions to insure the safety of its employees, customers and visitors to office locations," said a statement from the company.

Shares of both Citigroup (C: down $0.04 to $44.05, Research, Estimates) and Prudential (PRU: down $0.66 to $45.90, Research, Estimates) were slightly lower in morning trading Monday.