Rudy Giuliani offered some brutal “advice” for Mayor-elect Zohran Mamdani Monday — telling his successor he should never take office as leader of the Big Apple.

“My advice to him is to step down and do something else,” Giuliani told The Post of the proud democratic socialist who he branded a “communist — right out of Karl Marx.”

Giuliani, a Republican former two-term mayor of the city, blasted Mamdani as ill-equipped for the job and claimed he was a “supporter of Muslim extremism” after the Democratic Party nominee rubbed elbows with controversial Imam Suraj Wahhaj, a radical who had once urged a gun-free “jihad” on the city .

“The imam he was with is a strong supporter of terrorism,” Giuliani said, referring to a picture taken during the campaign of Mamdani — who will be the city’s f

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