City Council Speaker Melissa Mark-Viverito, in her State of the City speech, offered up criminal justice reforms Thursday that she said could sharply reduce Rikers Island’s population and realize the “dream” of closing the violence-marred jail complex.

But the leader of the correction officers’ union called the idea a “fantasy,” saying it could result in detainees being sent to jail facilities, several of them located in poorer neighborhoods.

Mark-Viverito named a commission to study how to cut pretrial detention rates and move adolescents and the mentally ill off Rikers and announced efforts to purge old summons warrants for petty crimes.

“Rikers Island has come to represent our worst tendencies and our biggest failures,” she said to an audience of hundreds at a South Bronx high school. They included the mother of Kalief Browder, who committed suicide after three years of pretrial detention at Rikers following his arrest at 16 on a charge of stealing a backpack.

“It is where Kalief suffered and his spirit broke down,” Mark-Viverito (D-East Harlem) said of the 400-acre jail complex that houses about 10,000 inmates. “We must explore how we can get the population of Rikers to be so small that the dream of shutting it down becomes a reality.”

City Correction Officers Benevolent Association president Norman Seabrook countered afterward: “That’s not a dream; that’s a fantasy. Because at the end of the day, where do you put these individuals that continue to commit crimes?”

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He said he agrees that young people shouldn’t be jailed for quality-of-life crimes such as breaking park rules and loitering after dark, but he urged the council speaker to “talk to the people that do this for a living as opposed to the ones that just push a pencil.”

A Mark-Viverito spokesman said the speaker seeks to shrink the number of detainees awaiting trial but not do away with the jail system. She envisions that Rikers’ higher-level offenders could be moved to smaller jails in the boroughs, where there could be improved oversight.

Mark-Viverito said her commission to “explore a community-based justice model” would be led by retired state court of Appeals Chief Judge Jonathan Lippman, a champion of bail and juvenile justice reform.

A spokeswoman for Mayor Bill de Blasio, who did not attend the speech, said the administration has already launched programs — such as fast-tracking gun cases — to shrink the number of Rikers detainees.

“Wherever we house our jails, we will continue reducing our jail population — as a matter of fairness, justice, and safety,” said Monica Klein, who did not directly address the idea of closing Rikers.

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A city Department of Investigation spokeswoman said more than two dozen correction staff have been arrested on charges of smuggling contraband and assaulting inmates since 2014.