N.Y.C. Votes to Close Rikers. Now Comes the Hard Part.

One 886-bed jail will tower over shops and restaurants in Downtown Brooklyn. Another will be next to a subway yard in Queens. In the Bronx, a jail will replace a Police Department tow pound. And another jail will rise in the shadow of City Hall in Manhattan.

That is at the heart of a plan for a landmark overhaul of New York City’s corrections system, which will culminate with the closing of Rikers Island, the jail complex with nearly 10,000 beds that has become notorious for chronic abuse, neglect and mismanagement.

The City Council decisively approved the proposal on Thursday, taking a step that seemed improbable just a few years ago. Supporters say the plan places New York City at the forefront of a national movement to reverse decades of mass incarceration that disproportionately affected black and Hispanic people.

Still, the aggressive timeline — closing Rikers by 2026 — could prove very challenging.

Mayor Bill de Blasio and Corey Johnson, the City Council speaker, embraced the jails plan, throwing their political weight behind the measure despite steady opposition from neighborhoods whose residents worry that towering new jails will harm their quality of life. Corrections officers also criticized the plan as unrealistic.

“What we are doing today will reshape the city for generations to come and impact the lives of every New Yorker,” Mr. Johnson said on Thursday. “For decades, our city was unfair to those who became involved in the justice system, and the overwhelmingly majority who were caught up were black and brown men.”

With two years left in office, Mr. de Blasio also secured perhaps his most progressive achievement so far as mayor with the vote to close Rikers — an idea that even he dismissed a few years ago as impractical.

“This is about valuing our people, no longer condemning people and sending them on a pathway that only made their lives worse and worse,” Mr. de Blasio said. “Today we made history: The era of mass incarceration is over.”

But now, the hard part begins. To meet its deadline of 2026, the city will have to build four new jails scattered in neighborhoods across the city, shut down a 400-acre jail network on an island in the East River, and relocate thousands of detainees and corrections officers.

The transition and new construction also carries a hefty price tag: more than $8 billion.

The city may find itself having to fend off the legal challenges that are often mounted when neighborhoods oppose major developments.

In a sign perhaps of the challenges and opposition ahead, Councilman Carlos Menchaca, who represents parts of Brooklyn, said the legislation did not go far enough to address the reasons people end up in jail.

“This vote only enriches developers in the short term,” said Mr. Menchaca, who voted against the proposal. “I do not trust this mayor, do you?”

To make the plan a reality, city officials must place their trust in future elected leaders to share the same vision and see it through on the same aggressive timeline. It also depends on further declines in the city’s crime rate, which by some measures is already the lowest it has been since the 1950s.

Today the city’s jails hold about 7,000 people every day — about a third of the population when it reached its peak during the crack cocaine epidemic in the early 1990s. The new jails, however, would not have room even for the number of people being held today.

The city announced this week that the new facilities will be smaller than first anticipated with a combined daily capacity of about 3,300 people, a number that the city has not recorded in nearly 100 years.

The jails will be about the same size — each with 886 beds — and will require reducing the jail population by more than half by 2026.

The Correction Officers’ Benevolent Association, which represents more than 10,000 correction officers in the city, has criticized the projected number of inmates as unrealistically optimistic.

The City Council’s minority leader, Steven Matteo, said on Thursday he had concerns over what it would take for the city to reach the goal of 3,300 detainees.

“It will require putting more potentially dangerous offenders back on the street, jeopardizing public safety,” said Mr. Matteo, a Republican from Staten Island who also voted “no.”

When the new jails open, officials said, they will be safer, smaller and more humane and will make New York’s corrections system a model for the rest of the country. Detainees will be provided job training, mental health counseling and education services.

Many of those incarcerated at Rikers, where people have been held since 1935, are awaiting trial and cannot afford bail. Many are grappling with mental health issues or drug addiction and critics say Rikers does a poor job of helping them overcome those challenges.

The new jails will also be closer to courthouses, eliminating a main complaint about Rikers, whose remote location places inmates far from their legal representatives and contribute to delays in court hearings that keep people in jail longer.

“I have been to Rikers. I have been to the Kew Gardens jail,” said Karen Koslowitz, a councilwoman who represents the area of Queens that would get a new jail. “They weren’t cells. They were cages.”

With the city moving to close the jail complex on Rikers Island, the nation’s second largest jail network, the future of the island is unclear. Mr. Johnson and other Council members are pushing legislation that would prevent the island from being again used to house jails.

While the new jails will be collectively smaller than those they replace, the individual facilities will be towering presences in their neighborhoods. The tallest jails will be in Brooklyn and Manhattan, which could both reach a maximum height of 295 feet tall.

Before the vote on Thursday, Mr. Johnson said that years of declining crime rates show that the inmate population would continue to drop and that the new jails would meet future needs.

“We feel confident this is the right size,” he said.

Smaller jails are possible, city leaders say, because of changes by New York State and the city to reduce the number of people sent to jail.

In recent years, New York State repealed the Rockefeller Laws, the 1970s antidrug legislation that established mandatory minimum prison sentences for nonviolent, low-level drug offenders.

New laws will go into effect in January that will outlaw cash bail for most people charged with misdemeanors and nonviolent felonies, and will require prosecutors to give evidence to the defense far earlier.

New York City has banned solitary confinement for people under 22 years old, expanded diversion programs that keep people out of jail and did away with arrests for most people found with small amounts of marijuana.

As part of the jail plan, the city also announced $265 million in new spending on efforts intended to keep people out of jail and to support those who have been released.

It would include an expansion of the supervised release program, which allows defendants to wait for trial outside jail; conflict-resolution lessons in schools; and transitional employment opportunities for inmates upon release.

Ydanis Rodriguez, a councilman who represents parts of Northern Manhattan and voted “yes,” said, “This plan is only the beginning, not the end.”

Jeffery C. Mays contributed reporting.