Mayor Michael Bloomberg, whose once-sky-high popularity has tanked in his third term, sought Thursday to burnish his legacy.

Speaking at the World Trade Center site in lower Manhattan, Bloomberg warned that the city's continued ascendence, which began in the aftermath of the 9/11 terrorist attacks -- its booming economy, its safe streets, its population growth -- is far from a fait accompli.

He recalled pundits wondering 12 years ago whether New York City would descend into the despair of the 1970s.

"The truth is, we have answered the questions about New York City's post-9/11 future -- but only for now," Bloomberg said during a Downtown Alliance breakfast meeting. "Because the day that we believe that our progress is inevitable, the day we take our safety and security for granted, the day that we believe that the past cannot repeat itself, is the day those questions will return.

"The future," the mayor said, "is not preordained."

Now, with the sun setting on his 12 years in City Hall, and most candidates seeking to succeed him trying to distance themselves from him, Bloomberg rattled off a laundry list of accomplishments: developing lower Manhattan, record low crime rates and preventing New York City from sustaining another terrorist attack.

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"It's critical that we continue investing in counterterrorism operations that are guided by the bounds of the law," Bloomberg said. "The threat we continue to face is not abstract, and it is not going away."

The mayor appeared to be referencing the NYPD's controversial mosque surveillance program criticized by Muslim and civil rights leaders.

Bloomberg focused on the booming neighborhoods of lower Manhattan, where, he said, 23,000 people were living the day before the attacks. Now, it's 60,000, the mayor said, adding "that number is steadily growing."