With Hurricane Irene pointed at the nation’s most populated city, expected to arrive by late Saturday, New York Mayor Michael Bloomberg wants coastal residents to get moving.

Now City officials made the unprecedented decision to shut down the mass transit system starting at noon Saturday and to evacuate by 5 p.m. more than 250,000 New Yorkers who live along the southern coastal rim of the city. That includes residents of Manhattan’s Battery Park City, vast parts of Staten Island, Brooklyn’s Howard and Manhattan beaches and everyone in the beachfront communities of Rockaway.

Even as Bloomberg acknowledged Friday’s sunny skies, he warned New Yorkers who live in the affected areas to get going — and for everyone else to make plans to spend most of Sunday indoors. All sporting, street and outdoor cultural events including a Dave Matthew’s concert are canceled for Satuday and Sunday, he said.

“The sun is shining but don’t be misled,” Bloomberg said at a news conference Friday. “There is a very dangerous storm heading in our direction…. It could slow or diminish in intensity, but there is no question we are going to get hit by some wind and high water that is very dangerous.”

The transit system, including subways, buses and trains, takes about eight hours to completely shut down — and about the same amount of time to restart, which could mean it won’t be functioning by Monday morning rush hour.

Jay Walder, the Metropolitan Transit Authority chairman, said it takes time to move the vast amount of equipment needed to cover a system that spans 5,000 square miles and is used daily during the week by about 8 million riders and by about half that number on the weekends. The entire system has only shut down on rare occasions, such as in blackouts.

New York has already been in high gear preparing for the fierce storm — evacuating hospitals, senior centers and psychiatric homes in low-lying areas, clearing many of the city’s 143,000 storm drains to avoid flooding and preparing city emergency workers for a long weekend.

For example, about 242 patients were transported from Coney Island Hospital to facilities away from the coast. The New York Police Department deployed about 50 small crafts to precincts along the shore in case people get stranded.

Bloomberg recommended that residents who are required to leave their homes find a friend or relative to stay with but that the city also would begin opening 91 emergency facilities at 4 p.m. Friday.

“We’ve never done a manadatory evacuation before and wouldn’t be doing it if it wasn’t necessary,” Bloomberg said, adding almost wistfully: “The best scenario would be if the storm veered off east.”

View Hurricane Irene track forecast in a larger map

Police will be patrolling the abandoned neighborhoods, Bloomberg said, advising people who hadn’t left to get going. But while stragglers could be fined for ignoring the evacuation order, the mayor said that probably wouldn’t happen.

“We don’t have the manpower to go door to door to drag people out of their homes…. Nobody is going to get fined, nobody is going to jail,” he said. But if they didn’t leave, he added, “people are just going to die.”

Decisions about closing the city’s many bridges and suspending the ferry system would be made as officials assessed the strength of the winds, which are expected to whip through at 36 miles per hour, officials said.

Walder urged New Yorkers who rely on mass transit not to wait to the last minute before it shuts down: “The buses will go a little bit longer (than noon) but I can’t stress enough: Please don’t wait for the last train. There is simply not the capacity for everyone to get on the last train … The sooner people make decisions to leave the better it will be for everyone.”

After the storm passes, Walder said New York’s Gov. Andrew Cuomo has offered the state’s helicopters to help identify damaged areas.

In fact, the cleanup was very much on everyone’s minds during the news conference: The mayor talked about falling trees, water towers toppling off apartment buildings, and windows shattering.

“It’s fun being out,” Bloomberg said. “But stay indoors…. Be safe.”