Hundreds of people were seen ignoring social distancing guidelines in Williamsburg on Tuesday night.
The funeral was held for Rabbi Chaim Mertz.
Video shows hundreds of people jammed together in the streets for the service.
In a series of angry tweets, de Blasio said this type of gathering will not be tolerated and the time for warnings has passed.
"We have lost so many these last two months and I understand the instinct to gather to mourn, but large gatherings will only lead to more deaths and more families in mourning," he wrote on Twitter. "We will not allow this."
He said he has instructed the NYPD to make arrests if necessary, however no arrests were reported.
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