There's mayhem in Manhattan, where a massive power outage has plunged a huge part of New York into darkness.

The blackout, just before nightfall on Saturday (local time) has affected more than 42,000 customers around Times Square and midtown Manhattan, CNN reported.

Thousands of people are on the streets with nowhere to go, with multiple subway stations in the dark and traffic light systems out, and people are reportedly stuck in lifts in their apartment buildings.

New York mayor Bill de Blasio says the outage appears to have been caused by a mechanical problem which triggered a number of transformer fires. They are not believed to be suspicious.

"This appears to be something that just went wrong in the way that they transmit power from one part of the city to another," he told CNN. "It sounds like it is addressable in a reasonable amount of time."

The power went out about 7pm.