MIAMI — In a startling denunciation of capital punishment in a state with one of the nation’s largest death rows, the new chief prosecutor in Orlando, Fla., said Thursday that her office would no longer seek the death penalty.

The decision by the prosecutor — Aramis D. Ayala, the state attorney for the Ninth Judicial Circuit — could have widespread repercussions in Florida if other prosecutors follow suit. Nationwide, support for the death penalty has steadily declined, along with the number of executions, and in Florida, years of litigation and legislative maneuvering have left the capital punishment system in turmoil.

Backlash from other law enforcement officials was swift. The state attorney general, Pam Bondi, called Ms. Ayala’s decision a “blatant neglect of duty.”

Within hours, Gov. Rick Scott issued an executive order removing Ms. Ayala from a case involving the death of an Orlando police officer.

Although the Florida authorities have not carried out any executions since January 2016, the state has 381 prisoners on death row. Twenty-two of them were convicted in Orange or Osceola Counties, the two counties within Ms. Ayala’s territory.

Ms. Ayala said that the death penalty had failed as a deterrent and that it did nothing to protect law enforcement officers. She also cited the length of time between sentencing and execution, which often exceeds a decade, and the costs of capital cases. “I am prohibited from making the severity of sentences the index of my effectiveness,” she said on Thursday. “Punishment is most effective when it happens consistently and swiftly. Neither describe the death penalty in this state.”

Seeking life sentences, she said, would guarantee that “violent offenders will never be released.”

“They will never continue to drain resources from this state with decades of appeals, and we can offer families of the victims more closure and more certainty,” she added.

As recently as Tuesday, Ms. Ayala’s spokeswoman told The Orlando Sentinel that the office was seeking death sentences in six cases. By late Thursday morning, the office had reversed its stand, throwing into question the potential punishment in a high-profile case against Markeith Loyd, who the police said killed his pregnant ex-girlfriend and then gunned down an officer, Lt. Debra Clayton, as she tried to apprehend him.

After Ms. Ayala’s announcement, Mr. Scott called on her to recuse herself from the case, but she declined. The governor then removed her from it, saying in a statement that she had “made it clear that she will not fight for justice.”

Ms. Ayala, the first black elected prosecutor in Florida, suggested that she had not made the death penalty an issue in her campaign last year because there was a hold on executions at the time.

“I do understand that this is a controversial issue, but what is not controversial is the evidence that led me to my decision,” she said. “If I require the attorneys in my office to manage cases — their individual cases — based upon evidence, I must follow the same evidence-based approach when I implement policies.”

Ms. Ayala’s new approach angered many people in local law enforcement, including some officials with whom she is expected to work closely. They said the case involving Mr. Loyd, who is representing himself and whose judge entered a not-guilty plea on his behalf, warranted a death sentence.

“If there was ever a case for the death penalty, this fits,” Chief John W. Mina of the Orlando police said in an interview, adding that he was commenting only on Mr. Loyd’s case. “If there is a reason we have a death penalty in the state of Florida, it’s for the more serious, heinous crimes, which need to have serious consequences. Anything short is disproportionate.”

Lieutenant Clayton’s family did not respond to a request for comment.

Death penalty critics welcomed Ms. Ayala’s new policy for her circuit, which has a population of about 1.6 million people and draws millions of tourists each year.

“A powerful symbol of racial injustice has now been discarded in Orange County,” Adora Obi Nweze, the president of the Florida conference of the N.A.A.C.P., said in a statement. “Ending use of the death penalty in Orange County is a step toward restoring a measure of trust and integrity in our criminal justice system.”

Mr. Scott’s decision to remove Ms. Ayala also provoked strong reactions.

“My God, is this a dangerous precedent,” said Howard Simon, the executive director of the American Civil Liberties Union of Florida. “Whenever the governor doesn’t like the exercise of prosecutorial decision by an elected prosecutor, he’s going to step in and appoint somebody else?”

Mr. Simon said the governor had engaged in “a very shortsighted abuse of authority.”

“‘Good and sufficient reason’ would mean an ethical problem or conflict of interest,” he said. “This is not the case here. The only reason he is using to remove the prosecutor is that he disagrees with the prosecutor’s judgment.”

Others worried about Ms. Ayala’s imposing a sweeping standard on all potential capital cases.

“You have some question whether or not the prosecutor is properly exercising prosecutorial discretion,” said George R. Dekle Sr., a former prosecutor and former University of Florida law professor. “I think any time anybody facing any kind of charging decision says ‘never,’ then they are not thinking too clearly.”

Ms. Ayala’s shift in policy came just days after Mr. Scott signed a law that was intended to resolve constitutional concerns about the state’s death penalty. That measure, which requires juries to reach unanimous agreement before imposing a death sentence, was the second time in two years that Florida tried to address court rulings that had cast doubt on capital punishment in the state.