New York ? A day after the nation was put on heightened terrorist alert, New York officials told residents to be vigilant but go about their business. Many New Yorkers did just that.

“You’ve got to live your life,” said Jonathan Marlow, 23, walking to his job at a midtown Manhattan investment banking firm.

But traffic on city streets, and particularly aboard subways, was sparse.

“This is amazing to me how empty it is,” said Elizabeth Bohlen of Mystic, Conn., waiting at Times Square to board a train.

Four uniformed police officers and a bomb-sniffing dog patrolled the station Saturday. Officers also were seen guarding popular tourist spots like the Plaza Hotel and Tiffany & Co. jewelry store, while three large police vans were parked at the southwest entrance to Central Park.

The Bush administration raised the national terror alert from yellow to orange Friday. Atty. Gen. John Ashcroft cited an “increased likelihood” that the al-Qaida terror network would attack Americans, noting hotels and apartment buildings were possible targets.

Stepped-up protections stretched across the country, from the San Francisco-Oakland Bay Bridge to today’s NBA All-Star Game in Atlanta. Nuclear facilities in California and Washington state and some utilities put security plans into action.

A high-ranking law enforcement source told The Associated Press that the measures came in response to intercepted communications between suspected terrorists, suggesting New York as a target.

Investigators were especially concerned about the possibility of chemical, biological or radiological attacks and information they received raised specific concerns about attacks on hotels and subways, said the source, who spoke on condition of anonymity.

Gov. George E. Pataki called up a special Air National Guard unit trained to handle bioterrorism and placed the state on orange alert. The city has remained on orange alert, the second-highest, since Sept. 11, 2001.

Security at New York's Grand Central Station is on heightened alert against possible terrorism. Police stepped up security at airports, subways and hotels on Saturday, a day after federal officials indicated New York was a possible terrorist target.

Security officials planned to enforce a rule at airports that bans public parking within 300 feet of any terminal buildings, said Mark Hatfield, a spokesman for the Transportation Security Administration. Additional passenger screening and random car searches would also occur at Kennedy International and LaGuardia airports.

Security also was boosted at river crossings in and out of the city. At the New Jersey entrance to the Lincoln Tunnel, a heavy presence of state troopers was evident, and trucks headed into the city were stopped and inspected, just as they were immediately after the Sept. 11 attacks.

Officials at hotels and private buildings wouldn’t disclose their additional security, but said they were taking precautions. Martine Roy, security manager at the Waldorf-Astoria hotel, said hotel and city police had both increased their presence.

Bohlen said she wasn’t aware of the alert until she read Saturday morning papers. She and her friend Dorothy Waglow, also of Mystic, Conn., said the threat wouldn’t stop them from going to the New York City Ballet.

“I have part of a subscription,” Waglow said, “so I wasn’t going to not go.”