San Francisco Mayor London Breed on Friday appointed Suzy Loftus as interim district attorney following George Gascón’s surprising resignation a day earlier, shaking up the race for the city’s next top prosecutor just one month before voters go to the polls.

The announcement was met with protest and accusations of a “power grab” and “political cronyism” from the three other candidates as Breed had already thrown her support behind Loftus. The American Civil Liberties Union, which does not support political candidates, blasted the move in a statement Friday, calling the appointment a “tilting of the scales” in the tight election race.

It’s unclear whether the mayor’s appointment will give Loftus a bump or ultimately backfire before voters go to the polls on Nov. 5.

“When I received the letter yesterday from the district attorney announcing his resignation, I couldn’t help but get excited about the future because I know that Suzy Loftus is the right person to do it,” Breed said at a press conference at the Far East Cafe in Chinatown. “We don’t just leave an office open just because there’s an election coming up.”

Loftus told the crowd she would “work every day to build safety that is not predicated on ZIP code.”

“I‘m accustomed to doing a lot at once, and I have an incredible team both within the D.A.’s office and across the city and in communities, who care about this race and care about that office and they’ll help me make sure I get the job done,” she said.

Breed’s team moved the announcement indoors after a planned news conference at Portsmouth Square two blocks away was interrupted by a small but raucous group of protesters who shouted back and forth with Loftus’ supporters over bullhorns.

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The mayor has said repeatedly in recent months that she needs a district attorney who she can “work with” to accomplish her agenda — a thinly veiled jab at Gascón and a window into what appears to be the icy relationship that developed between them. Loftus’ appointment comes as Breed parries mounting criticism over the city’s slow response to street-level problems.

Gascón on Thursday upended the district attorney’s race when he announced his planned Oct. 18 departure ahead of his move to Los Angeles, where he is exploring a run for the same job. He previously said he was not seeking re-election before his term was set to end Jan. 1, allowing for the first open field for the position in more than a century.

The four candidates in the race — Loftus, Chesa Boudin, Nancy Tung and Leif Dautch — were all hitting the homestretch of their campaigns, but the sudden resignation and appointment is expected to become a central focus the rest of the way.

Loftus is assistant chief legal counsel at the Sheriff’s Department and served as president of the Police Commission during a turbulent period of fatal police shootings in San Francisco. She previously worked under Kamala Harris in the state attorney general’s office and at the city district attorney’s office.

Friday’s move wasn’t a complete surprise to Loftus’ opponents. Breed signaled on Thursday she would likely appoint someone rather than let voters have the first say in deciding Gascón’s successor.

Still, when news of the appointment broke Friday morning, the three other candidates slammed the mayor’s decision and accused her of attempting to meddle in the election.

“This is obviously a power grab by an establishment that is determined to protect itself rather than the people of San Francisco,” Boudin said. “Voters will see this is about politics, not public safety. I’m confident our campaign can overcome this last-minute effort to preserve the status quo. San Francisco voters want a D.A. that’s accountable to them, not the mayor.”

Dautch said voters “deserve the right to choose their next district attorney on a clean slate.”

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“Machine politics caused the problems facing our city, and machine politics won’t solve them,” he said.

Tung said the appointment “smacks of political cronyism.”

“This is not about leadership in the D.A.’s office. It’s about artificially boosting her political best friend when we’re so close to an election,” she said. “To take away this historic election from the voters, I think is a little bit disrespectful to the people of San Francisco. It very well may backfire.”

Breed’s appointment, though, was welcomed by the city’s police union, which ardently opposes the prospect of Boudin as the next district attorney. Boudin is an attorney in the public defender’s office and his parents were jailed for murder following a bungled armored car robbery that left three dead — including two police officers — in 1981.

In a recent political twist, the San Francisco Police Officers Association is now showing support for Loftus, who it clashed with during her time on the Police Commission when she worked to amend the department’s use-of-force policy.

“We respect Mayor Breed’s authority to appoint a district attorney, and we are thankful it is not the criminal and domestic-terrorist apologist who is running to replace the failed Gascón,” said Tony Montoya, the union’s president.

The appointment isn’t the first time Breed has flexed her political muscles and given her preferred candidate a bump. Last year, she appointed Faauuga Moliga to a vacant seat on the San Francisco Board of Education, clearing his path to victory three weeks before election day.

Incumbency could cut both ways for Loftus, and it was not immediately clear how she intends to juggle the homestretch of her campaign with district attorney duties. The move significantly boosts her public visibility but also invites considerable scrutiny. Attempts at making any major changes to the office’s approach to prosecuting cases in such a short amount of time could prove risky.

“Decisions with ramifications on people’s lives and liberties aren’t always going to be popular,” said Max Szabo, a longtime aide and spokesman for Gascón who recently left the office. “Running for D.A. is a popularity contest. Being D.A. is anything but.”

Voters could also take issue with Breed attempting to give Loftus an unfair advantage in the race.

“I was very concerned to hear that the mayor was expected to appoint one of the candidates for D.A. a month before the race,” said Tab Buckner, a resident of the city’s Haight-Ashbury neighborhood who attended Friday’s announcement. “If the mayor is going to have somebody temporary, fine. But to choose a favorite candidate a month before the election is an abomination of the democratic process.”

Not appointing Loftus, however, could have read as “a vote of no-confidence” in her preferred candidate, said Nate Ballard, a political strategist who is supporting Loftus and previously served as an aide to then-Mayor Gavin Newsom. Loftus supported Breed in her election, and the mayor returned the favor when Loftus announced her candidacy more than a year ago.

Boudin, who is running to the left of Loftus, is part of a groundswell of progressive and activist candidates vying for district attorney positions around California and the rest of the country.

Tung said she would be a progressive district attorney, but is running the most moderate campaign of the group. A former prosecutor in San Francisco who now works in Alameda County, Tung is running on a platform focused on cracking down on the open-air drug markets in neighborhoods like the Tenderloin.

Dautch is a deputy attorney general in California and first-time political candidate running on issues like environmental justice and preventing fraudulent evictions. He’s the former president of the San Francisco Juvenile Probation Commission and is pushing a plan to turn the city’s juvenile hall into a mental health justice center.

Editor’s note: A previous version of this story incorrectly stated that Breed is running unopposed.

Evan Sernoffsky and Dominic Fracassa are San Francisco Chronicle staff writers. Email: esernoffsky@sfchronicle.com, dfracassa@sfchronicle.com Twitter: @EvanSernoffsky @DominicFracassa