NEW YORK, Aug. 1 -- Wall Street firms on Sunday began increasing already tight security but planned to open as usual Monday morning after warnings of a possible al Qaeda strike against targets in the financial industry.

The threat warning from the Homeland Security Department listed the New York Stock Exchange and Citigroup Inc. buildings in Manhattan and the Prudential Financial Building in Newark as targets al Qaeda plans to attack. The warning also listed the World Bank and the International Monetary Fund in the District, where officials also said they would open as usual Monday. The stock exchange, with a colonnade exterior that serves as the iconic face of American capitalism and where billions of dollars in stock transactions occur each weekday, is guarded by heavily armed police officers and has been closed to the general public since the Sept. 11, 2001, terrorist attacks at the Word Trade Center and Pentagon. Those attacks sent world markets tumbling and shuttered the NYSE for four trading days, the longest period since the Great Depression.

When trading reopened in New York on Sept. 17, special rules designed to stop a market free-fall were put in place, including allowing major corporations to buy back more of their own stock than ordinarily allowed. Despite the rules, the Dow Jones industrial average, a major stock market bellwether, plunged 684.81 points when the market reopened, its biggest point drop in history.

In another action to prop up the markets after the 2001 attacks, the Federal Reserve added more than $100 billion into the U.S. banking system to ensure that brokerage firms, investment banks and other companies had no trouble financing loans or other types of investment activity.

The NYSE now maintains a backup facility at an undisclosed site that could take over trading within a day if the main building in lower Manhattan is damaged, exchange officials have said. The exchange declined to comment Sunday on any security changes after the new threat alert. New York City Mayor Michael Bloomberg (R), who on Sunday urged New Yorkers to be vigilant but otherwise go about their lives as usual, announced plans to ring the opening bell at the NYSE Monday morning.

Homeland Security Secretary Tom Ridge said al Qaeda wanted to attack high-profile financial targets using a car or truck bomb. New York officials said they have also warned the heads of security for major Wall Street firms to monitor their heating and cooling systems to guard against a possible chemical attack.

NYSE chief executive John A. Thain sent an e-mail to employees Sunday saying he had spoken with Bloomberg and Ridge.

"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," the e-mail said. "The Exchange will continue to be open for business on our normal schedule. The safety of our employees, members, and everyone who works in the NYSE facilities is our number one priority and is of the utmost importance to us. Please be reassured that we remain vigilant and will keep you updated."

Thain plans to greet employees arriving at exchange Monday , along with Sen. Charles E. Schumer (D-N.Y.). About 4,500 people work at the exchange.

In addition to adding police at the NYSE, the New York police department said it would increase protection at buildings owned by Citigroup, especially its 59-story skyscraper headquarters, built in 1978, in midtown Manhattan. Citigroup, the nation's largest financial services firm with major operations in both commercial and consumer banking, has 25,400 employees in New York City. The employees received an e-mail Sunday saying security would be increased in the coming days.

'[Citigroup] makes every effort to ensure your safety at all times and particularly in light of recent reports of possible threats and the upcoming Republican National Convention in New York City," said the e-mail, from the head of the firm's internal security and investigations unit. "In the coming days and weeks, you can expect to see additional security measures being implemented." The e-mail did not say whether employees should avoid their offices, but a Citigroup executive said it would be "business as usual" at the company.

The Prudential Financial headquarters building in Newark will be open for business Monday, spokesman Michael Hanretta said. The 24-story headquarters facility houses 1,600 employees and 1,200 more work in a nearby office.

"Prudential is already coordinating with local, state and federal officials and has taken additional security precautions to ensure the safety of its employees, customers, and visitors to all of its locations," Hanretta said. The firm shared information with employees Sunday but declined to make those communications public.

Prudential's flagship building already had been surrounded by security barricades. On Sunday, Newark police moved to set up metal fences and block two surrounding streets. An emergency response team, armed with assault rifles, was stationed outside the building. Todd McClendon, a Newark police detective, declined to say how long the police would continue their "heightened presence."

Spokesmen at many New York investment banks, including Bear, Stearns & Co., Goldman Sachs, and J.P. Morgan Chase & Co. declined comment or did not return calls on Sunday. All of the major firms have significantly increased security in recent years, requiring visitors to show photo identification and have their bags searched. They have also developed backup trading facilities outside Manhattan. One large firm, Morgan Stanley, plans to test its backup facilities during the Republican National Convention, which takes place at Madison Square Garden from Aug. 30 to Sept. 2. Since the 2001 attacks, government agencies and securities industry representatives have met repeatedly to plan conducting business after another terrorist attack. The NYSE and the New York-based Nasdaq Stock Market each agreed to serve as backups for each other if a catastrophe struck one of the markets.

Securities and Exchange Commission official Robert L.D. Colby told Congress last year that the agency was mostly concerned about the process of confirming trades and ensuring sellers are paid and buyers receive their securities. He said redundancies in technology and information sharing had helped minimize the importance of any one stock market or securities firm in the process.

White reported from New York and Johnson from Washington.