Officials say a 51-year-old woman was walking down the street on East 47th Street and Third Avenue just after 2 p.m. when she was approached from behind by a male suspect.
Police say the man hit the woman in the back of the head with a cinder block before fleeing westbound on East 48th Street.
The NYPD says this was an unprovoked attack.
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The victim sustained injuries to the back of her head.
She was taken to Bellevue Hospital in serious but stable condition.
Police released surveillance video and pictures of the unidentified suspect on Thursday night.
There are no arrests at this time.
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