Entire units of inmates are left unguarded. Suicide attempts among prisoners are rising. Nearly a third of correctional officers are out sick or can’t work with prisoners.

New York officials are scrambling to address alarming conditions at the Rikers Island jail complex, one of the nation’s largest detention centers. Lawyers and lawmakers say the safety of inmates and officers has never been more imperiled in the 90-year history of Rikers, long known for its reputation for violence.

“The...

Entire units of inmates are left unguarded. Suicide attempts among prisoners are rising. Nearly a third of correctional officers are out sick or can’t work with prisoners.

New York officials are scrambling to address alarming conditions at the Rikers Island jail complex, one of the nation’s largest detention centers. Lawyers and lawmakers say the safety of inmates and officers has never been more imperiled in the 90-year history of Rikers, long known for its reputation for violence.

“The humanitarian crisis I saw is, simply, stunning,” New York City Public Advocate Jumaane Williams said after a tour of Rikers last week.

New York Gov. Kathy Hochul on Friday ordered the release of 191 individuals from Rikers and signed a bill that aims to end reincarceration for technical parole violations. The law, known as the Less is More Act, will help lower the inmate population at Rikers, improving conditions, city and state officials said

“No one—no inmate, no correction officer, no family member who visits—should have to witness the reality of Rikers as it exists today,” the governor said, calling the jail’s current conditions “an indictment on everyone.”

A host of problems are causing the situation at Rikers, city officials say.

The jail complex was hit hard by Covid-19. At the start of the pandemic, Rikers infection levels were among the highest of any known workplace in the U.S.

Since then, the jail has seen simultaneous staffing shortages and a steadily rising inmate population.

More than 2,600 of New York City’s 8,400 uniformed correctional workers are out sick or on medically modified duty, which means they can’t work with prisoners, according to city officials. Others have simply not shown up, the officials said. Officers who do report to work have been forced to work as many as four back-to-back shifts, often without access to food or rest.

Meanwhile, the inmate population at Rikers has grown to 5,800 in August, compared with 4,000 in the early months of the pandemic.

Covid-19 has also forced the courts to scale back operations, leading to lengthier stays at the jail, which primarily holds people awaiting trial, New York City Department of Correction Commissioner Vincent Schiraldi said at a City Council hearing. Long stays have more than doubled during the pandemic, he said, contributing to crowding and violence.

“No one is supposed to be here for several years,” he said.

Ross MacDonald, the chief medical officer for Correctional Health Services, sought help from City Hall this month to address what he called “a collapse in basic jail operations.”

“I do not believe the city is capable of safely managing the custody of those it is charged with incarcerating,” Dr. MacDonald wrote.

New York City Mayor Bill de Blasio said the city would seek to discipline malingering officers.

“’When officers fake being sick and stay out inappropriately, when they’re AWOL, they’re hurting all their fellow officers, they’re hurting everyone,” Mr. de Blasio said Friday on the Brian Lehrer Show.

Benny Boscio Jr., president of the Correction Officers’ Benevolent Association, the union representing 7,600 active officers, criticized Mr. de Blasio’s response, calling him a “modern day dictator.”

Earlier this summer, the union sued the city’s departments of Correction and Labor, alleging inhumane, unsanitary and dangerous working conditions at Rikers. There has been a 23% increase in assaults on officers, according to the mayor’s 2021 management report. Around 1,200 officers have resigned during the past two years.

“What was once considered a solid career path is now a daily nightmare for the men and women who work tirelessly to keep this city safe,” Mr. Boscio said.

Conditions for inmates have deteriorated quickly, said defense lawyers and advocates of criminal-justice overhauls. According to data from Correctional Health Services, which provides services to inmates, there have been 539 incidents of prisoners hurting themselves between April and June, bumping the rate to 95 incidents per 1,000 detainees, the highest rate in five years.

Ten people in Rikers custody have died in the past year, including five who took their own lives. Suicide attempts are happening with frequency, too.

During a visit to Rikers last week, Mr. Williams said he observed inhumane conditions: detainees using a communal bag as a toilet; inmates housed in showers without the ability to sit or lie down; and people who had been denied access to meals, to medication, to phones and to transportation to court.

Public defenders at the Legal Aid Society received reports that inmates were left unattended in housing units, with no staff to lock cell doors, take them outdoors or bring food and water.

In a statement Friday, Mr. Schiraldi welcomed the visits from elected officials and advocates, who “now have a firsthand picture,” he said.

Concerns about Rikers Island arrive as uncertainty surrounds the future of the city’s jails. In 2019, the City Council approved a sweeping $8 billion plan to overhaul New York’s correctional system and close Rikers Island by 2026, replacing it with a network of smaller jails spread across the city’s five boroughs.

Mr. de Blasio, a Democrat with four months left in office, championed the effort to close Rikers, but the plan—a defining piece of his legacy—will largely be left in the hands of the city’s next mayor.

Write to Deanna Paul at deanna.paul@wsj.com