Aisling Tomei, a TV host in Canada who was in NY for the weekend, also shared video of citizens directing traffic as the hours wore on and the city grew darker.
"From what we're seeing at this moment, this is simply a mechanical problem and one, again, that sounds like it is addressable in a reasonable period of time", de Blasio said.
The New York City Fire Department said the cause of the outages, which have left more than 45,000 Con Edison customers without power, was a transformer fire which started at West 64th Street and West End Avenue.
NYPD and FDNY are on scene, the NYPD announced on Twitter.
The AMC Lincoln Square 13 movie theatre on the Upper West Side, the entire Cineplex was evacuated. Radio City Music Hall's facade was dark as well.
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Con Ed has reported that over 44k customers are without power in Midtown Manhattan.
While the company is still working on an official cause, a spokesman said the outage may be due to a possible equipment failure at a substation. The singer was 30 minutes into her concert at Madison Square Garden when the venue house lights came on.
The blackout seems to extend as far north as 72nd Street, according to Con Edison.
While trains are still running despite the outage, the stations without power are being bypassed by trains until power is restored. There was also "smoke in multiple buildings" on the West Side, the department said, and it was responding to "numerous" reports of people stuck in elevators. The Nos. 1, 2, and 3 trains are all running on the local track in Manhattan.
On Twitter, Con Edison described the outage as "extensive". He said the A, C, F, D and M subway lines were affected.
Saturday marked the 42nd anniversary of the 1977 blackout, which affected much of the city for 48 hours and resulted in widespread looting, arson and other criminal activities.