In a stunning upset, City Council candidate David Greenfield — who was backed by Mayor Bloomberg — trounced his opponent, Joe Lazar, in a race for a Brooklyn seat tonight.

Greenfield, a longtime political fixture in the Orthodox Jewish community he was elected to represent, secured 58 percent of the vote to 39.5 percent for Lazar.

Lazar was backed by Assemblyman Dov Hikind (D-Brooklyn) as well as powerful Assembly Speaker Sheldon Silver (D-Manhattan) — and had more momentum heading into the special election.

But Greenfield stumped through the Borough Park neighborhood last week with Bloomberg. The mayor helped pull off the win despite his cooling relations in recent years with Brooklyn’s Orthodox Jews.

The seat had been held by Simcha Felder, a Democrat, who won re-election last year.

He resigned to become a deputy comptroller for newly elected John Liu.