Fearing potential release of hundreds of convicted criminals, city and law enforcement officials announced immediate plans Thursday to put more specialized police units on the streets, saying the deepening fallout from suspected evidence tampering at a state lab demands aggressive action.

City officials said “crisis reentry” teams of police, probation officers, and street workers will work with freed offenders to help them reenter society and send a message of “zero tolerance” for criminal activity.

“These are not low-level drug users,” Suffolk District Attorney Daniel F. Conley said. “These are people with violent histories across the board, who are trafficking large amounts of drugs. This is a tough group of people.”

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More than 500 prisoners convicted in Suffolk County, many with long histories of drug-dealing and violence, could have their convictions stayed or revoked and be set free, according to officials. At least 100 federal cases could be affected.

The cases involved evidence tested by Annie Dookhan, the former state chemist who has allegedly admitted to mishandling drug evidence for the last two or three years. But even cases in which she may not have personally tested the evidence are being challenged.

In the first public description of the inner workings of the lab, a co-worker testified Thursday that Dookhan had virtually unrestricted access to the drug-testing laboratory in Jamaica Plain. The chemist, who did not testify, had the codes to gain access to the lab’s computer system and could open the lab in the morning, her colleague said.

Daniella Frasca, who worked next to Dookhan for eight years, was testifying in the trial of Shawn Drumgold, an accused drug dealer. His lawyer is trying to discredit the drug evidence by probing the lab’s workings.

The drug lab scandal has thrown the state’s criminal justice system into turmoil, potentially affecting 34,000 cases.

Its impact on federal cases began to come into focus Thursday. Federal officials said they had determined that Dookhan handled drug samples in at least 100 federal cases and possibly twice that many. Prosecutors and defense lawyers are reviewing whether her involvement corrupted those cases.

The alleged evidence tampering may also threaten cases in which federal prosecutors used state drug convictions to trigger tougher sentences for repeat offenders.

“That’s a whole larger universe of cases in which the laboratory did the testing,” said US District Chief Judge Mark Wolf, who met with other judges this week to discuss the scandal. “That’s a number I don’t think anyone can estimate right now.”

The reentry teams begin meeting Friday with inmates slated for release. Ed Davis, commissioner of the Boston Police Department, said the teams meet with small groups to warn them they will be under close watch.

“If you are released and re-engage in criminal activity, you will be arrested,” Davis said.

The plan is modeled after the Boston Reentry Initiative, a partnership between prosecutors and Boston law enforcement that provides released inmates with mentoring, job training, and addiction treatment.

City officials said they will also deploy more drug and gang units in hope of deterring crime. The prospect of so many inmates being released in such a short amount of time was alarming, they said.

“This is an emergency situation,” said Mayor Thomas M. Menino. “. . . We are concerned about the large number of individuals who will be released from state prison with no plan for transition back to society. And we’re just as concerned about those who may return to a lifestyle that can cause turmoil and crime in the streets of our city.”

The effort will cost an estimated $3.5 million, and officials called for state and federal assistance to offset the expense, given the magnitude and suddenness of the crisis. Prosecutors had earlier asked the Patrick administration for $10 million a year to handle new legal expenses for thousands of cases.

“We’ve never seen anything like this,” said Conley, whose office serves Suffolk County, where a dozen prisoners have already been released because of potentially tainted drug evidence.

Conley said prosecutors will seek to keep the most serious offenders incarcerated and noted that many pleaded guilty to charges that did not involve drugs, which could keep them behind bars. For those who are released, prosecutors will request GPS monitoring and curfews. In some instances, they will seek to retry cases.

Last month, a Suffolk County judge stayed the 10-year sentence of a career criminal on drug-trafficking charges, even though he was also convicted of gun violations, Conley’s office said.

On the federal level, Wolf said the scandal underscores judges’ concerns that lower-level drug cases are clogging up the federal court system.

“To me, it’s disturbing that the work of a state Department of Public Health lab has the potential to raise questions about the validity of decisions in a large number of cases in the United States District Court and that significant resources will have to be diverted to address any such issues,” he said.

US Attorney Carmen M. Ortiz defended the prosecution of the cases in federal court, saying FBI and Drug Enforcement Administration agents worked with local police to prosecute the most dangerous and violent offenders.

“These are individuals who are causing the greatest amount of danger to these communities,” she said.

But the American Civil Liberties Union and Families Against Mandatory Minimums called for dismissal of thousands of minor drug cases, as well as major drug prosecutions where defendants have served at least half their prison sentence.

“Abundant evidence shows that prosecuting drug cases takes a terrible human and financial toll even in cases that do not involve tainted evidence,” said Matthew Segal, legal director of the ACLU in Massachusetts. “It is not time for an expensive, misguided, and ill-fated do-over.”