The second massive storm to threaten New York City in 14 months will shut the city down Monday, with public schools and transportation closed and more than 375,000 residents forced to flee flood-prone areas.
He said those who want to ride out Hurricane Sandy won't be arrested, but they will be putting themselves and first responders in unnecessary danger.
"The biggest fear is that people don't leave," Bloomberg said during a news conference Sunday on storm preparations. "They call emergency workers and the emergency workers lose their lives trying to save others."
The mayor's words of warning came shortly after Gov. Andrew M. Cuomo ordered the closing of the city's mass transit system starting at 7 Sunday night.
Bloomberg and Cuomo made the same decisions in late August 2011 when Hurricane Irene took aim at lower Manhattan before it weakened.
Residents not living in flood areas Sunday flocked to hardware stores, supermarkets and other shops for final preparations.
The mayor encouraged evacuees to stay with friends and family in areas of the city on higher ground. For those who had nowhere else to go, Bloomberg and other officials said, 76 shelters were set up at schools and other buildings across the city.
By Sunday afternoon, many of those shelters saw long lines as families big and small came to sleep on cots.
Denise Kilbrug, 31, arrived at Seward Park High School on Grand Street near the Williamsburg Bridge in Manhattan with her husband, 8-month-old son and their dog, Dodger.
They had to evacuate their Battery Park City apartment.
Although the family had prepared for this over the weekend, they said they had only a limited amount of time to get everything done.
"It's inconvenient. We only packed so many bottles and canned food," Denise Kilbrug said.