Michael Bloomberg on Wednesday used his last major speech as mayor to declare that an "urban renaissance" is well underway -- and to warn it could be undermined if his successor doesn't stand up to municipal unions and halt the "explosion" of pension and benefit costs.

Without mentioning Mayor-elect Bill de Blasio by name, Bloomberg said the city's 152 expired labor contracts give the incoming administration a "once-in-a-generation opportunity for comprehensive benefit reform, which is essential to our long-term future."

Bloomberg said pension costs alone have ballooned to $8.2 billion from $1.5 billion a year during his 12-year tenure.

With health benefit expenses also skyrocketing, Bloomberg said, "The costs of today's benefits cannot be sustained for another generation -- not without inflicting real harm on our citizens, on our children and our grandchildren."

Bloomberg, whose term ends Dec. 31, spoke at an Economic Club of New York lunch in midtown Manhattan -- the second stop of a five-borough legacy tour highlighting his administration's successes.

Criticizing the political power of governmental unions, the billionaire mayor said a "labor-electoral complex" has stymied reform.

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The phrase was reminiscent of former President Dwight Eisenhower's often-cited farewell warning about the power of the "military-industrial complex."

De Blasio bristled at Bloomberg's remarks, saying no other mayor in the city's history -- whatever his political ideology -- has left office with all contracts open.

"So I would caution -- much as I appreciate Mayor Bloomberg's advice -- I would caution that one should be careful about giving advice from that perspective," de Blasio said in Brooklyn as he named a budget director, Dean Fuleihan, to handle contract negotiations, among other financial tasks.

De Blasio called the next four years a "once-in-a-lifetime opportunity to do a lot of things -- to address inequality, to fundamentally change the way New York City government relates to its people and certainly to put us on a strong fiscal footing for the future."

De Blasio said Bloomberg was simply trying to "cement his legacy."

Bloomberg said the city under his watch has led the world in innovations in such fields as education and economic development.

"It's safe to say that it's clear that the golden age of the suburb is over, and it is being replaced by a new urban renaissance that is redefining the future," he said.

Teachers who once left New York City in droves for better jobs in Nassau and Westchester counties are now staying in the city, he said.

He said he had sought to be forward-looking as a mayor.

"Tomorrow is more important than today," he said.

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With Matthew Chayes