About 5,500 of roughly 70,000 spots in New York City's emergency-evacuation shelters had been filled as of Saturday evening, raising serious questions about whether New Yorkers are complying with a mandatory evacuation order affecting roughly 370,000 residents in low-lying areas.

Mayor Michael Bloomberg said his administration has observed a "marked increase" in the number of people evacuating since Saturday morning. Still, he voiced grave concern that a significant number of people will defy his unprecedented order to evacuate ahead of Hurricane Irene's arrival.

The mayor ordered residents to leave so-called "Zone A" areas, plus the entirety of the Rockaways in Queens, by 5 p.m. Saturday. City officials said they don't have any data on how many people have complied, but the mayor and others conceded there is reason for concern. New York's mass-transit system shut down on Saturday afternoon, adding another challenge for evacuees.

Separately, New York Gov. Andrew Cuomo visited the 69th Regiment Armory on Manhattan's Lexington Avenue to announce he was deploying 1,000 National Guard troops, in addition to the 1,000 already deployed.

"The National Guard will play a vital role in this situation, Mr. Cuomo said.