Updated at 2:23 p.m. ET
NEW YORK - New York City Mayor Michael Bloomberg has ordered mandatory evacuations for residents in low-lying coastal areas ahead of Hurricane Irene.
Bloomberg said at a briefing Friday that emergency shelters will be opened at 4 p.m.
The low-lying areas are scattered across the city and are home to about 270,000 residents. They include parts of Battery Park City, Coney Island and the Rockaways.
Bloomberg said the mandatory evacuations are a first.
Officials also ordered an unprecedented shutdown of the city's mass transit system for Saturday in advance of the hurricane.
The Metropolitan Transportation Authority has never before halted its entire system in advance of a storm, though the system was seriously hobbled by an August 2007 rainstorm. The last planned shutdown of the entire transit system was during a 2005 strike.
New York Gov. Andrew Cuomo announced the transit shutdown will start at about noon on Saturday. The shutdown will include the city subway system, Long Island Rail Road, Metro-North Railroad and Access-A-Ride.
A hurricane watch is in effect for New York City and Long Island for Sunday, with storm conditions possible Saturday night.
Cuomo's office says major bridges in the city and lower Hudson Valley and the state Thruway will close if sustained wind speeds top 60 miles per hour.
As Hurricane Irene rumbles steadily closer the city, some evacuations have already begun, CBS News station WCBS-TV reports.
Staten Island University Hospital and Coney Island Hospital have begun evacuating patients to vacant beds at medical centers on higher ground. NYU Hospitals Center and Veterans Administration Medical Center are the other hospitals in the evacuation zone, according to the New York City Office of Emergency Management.
Bloomberg ordered some of those hospitals, nursing homes and senior centers in the most flood-prone areas of the city to be evacuated by 8 p.m. ET Friday.
If those facilities choose not to evacuate, they would need to coordinate their decisions in conjunction with city Health Commissioner Thomas Farley, Bloomberg said.
At Staten Island University Hospital, the evacuation began Thursday night. The first ones out: Newborns in the neonatal unit, who were taken to Cohen Children's Hospital in Lake Success, Long Island, the Staten Island Advance reported.
The Staten Island hospital was scheduled to begin the evacuation of its general population Friday morning. Patients will be discharged when possible or sent to a variety of other hospitals and nursing homes that are part of the North Shore-LIJ system. That includes hospitals in Manhassaet, Plainview, Syosset and Huntington on Long Island, the Advance reported.
"What we have to do is assume the worst, prepare for that, and hope for the best," Bloomberg said in statement to reporters Thursday evening.
Bloomberg also revoked all permits for planned outdoor events Sunday, when the worst of Irene's wrath is expected to hit the area.
The World Trade Center construction site in lower Manhattan is also making preparations for Irene. The Port Authority of New York and New Jersey said it is securing all cranes and construction gear at the site.
Also, New York City Buildings Commissioner Robert LiMandri has inspectors checking construction sites throughout the city for debris and loose material.
Urban planner and architect Lance Jay Brown said the scaffolding that adorns New York City sidewalks should be secure if it has been built to code specifications. Air conditioners should also hold in place. The greatest danger is from loose materials that could be turned into projectiles in the howling wind.
Windows above the 10th floor are at greatest risk of shattering. Terrace furniture and other loose things on rooftops could be dangerous.
Irene actually weakened a bit overnight, dropping to a Category 2 storm. That means it is packing winds of around 105 mph. It is still barreling toward North Carolina. It is possible that it will gather strength again.
By the time Irene hits New York, it is expected to bring at least tropical storm-force winds of at least 75-90 mph and up to 7-10 inches of rain.