Hundreds of thousands of New York City area residents were ordered to leave their homes Sunday ahead of Hurricane Sandy, a dangerous storm that forced the closure of the nation's largest mass transit system and upended daily life for millions.

Even before Sandy's expected landfall early Tuesday morning, the threat of high winds, storm surges and a deluge of rain rippled through Greater New York. The Metropolitan Transportation Authority halted train, bus and subway service throughout the region, airlines canceled thousands of flights and coastal communities from the Jersey Shore to eastern Connecticut moved hospital patients, closed schools and urged residents to seek shelter or higher ground.

Just 14 months after Tropical Storm Irene devastated many surrounding communities but largely spared the city, government officials and many residents said they were bracing for a far more serious blow. President Barack Obama granted New York's request for a federal emergency declaration on Sunday.

"This is the largest threat to human life our state has experienced in our lifetime," said Connecticut Gov. Dannel Malloy.

Power companies across the region said they were prepared for the storm, but elected officials predicted that hundreds of thousands could lose power for days. Utility workers were on high alert in New York state, Gov. Andrew Cuomo said, and Con Edison said it would have crews in the city prepared to disable underground equipment in the event of serious flooding.