NEW YORK (Reuters) - New York City on Friday ordered the evacuation of more than 250,000 people and prepared to shut down its entire mass transit system ahead of Hurricane Irene's expected arrival over the weekend. "We've never done a mandatory evacuation before and we wouldn't be doing it now if we didn't think this storm had the potential to be very serious," Mayor Michael Bloomberg told a news conference. The unprecedented evacuations were ordered for people living in low-lying areas near the waterfront, and police had a fleet of rescue boats at the ready. Homebound elderly and hospital patients in low-lying areas began evacuating earlier on Friday. At Coney Island Hospital, ambulances were transporting 250 patients to other hospitals ahead of a shutdown set for 8 p.m., said Evelyn Hernandez, a hospital spokeswoman. President Barack Obama on Friday warned Americans to take Hurricane Irene seriously and urged them to obey orders to evacuate from the path of what is likely to be an "extremely dangerous and costly" storm. "All indications point to this being a historic hurricane," Obama said in a statement to reporters from the farm where he is vacationing on an island off the Boston coast. Fifty-five million people are potentially in Hurricane Irene's path, from the Carolinas to Cape Cod on the US East Coast, and tens of thousands are evacuating as cities including New York brace for the powerful storm to hit. "I cannot stress this highly enough. If you are in the projected path of this hurricane, you have to take precautions now. Don't wait. Don't delay," Obama said. Obama and his family will leave Martha's Vineyard on Saturday, well before Irene is projected to scour its way this far north on the New England coast. But the vacation had already been marked by natural disaster, after the island felt an earthquake in Virginia on Tuesday that shook Washington and New York. In the low-lying Financial District surrounding Wall Street, the New York Fed was readying contingency plans for the storm and expected normal functioning of its open market operations on Monday, a spokesman said. The New York Stock Exchange was preparing a backup power generator and bringing in extra fuel and food to avoid disruptions when trade resumes on Monday. New York Governor Andrew Cuomo said 1,000 National Guard soldiers and airmen would help over the weekend and that the city's mass transit system, which carries 8.5 million people a day, would begin a system-wide shutdown starting at noon on Saturday.