William J. Bratton's announcement Wednesday that he would resign as chief of the Los Angeles Police Department caught Angelenos by surprise, including the mayor and police leaders who suddenly found themselves confronted with the daunting task of replacing one of the nation's most influential law enforcement figures.

Bratton's unexpected decision set in motion what promises to be an intense and wide-ranging search for his successor. With just three months before Bratton departs for his new job as head of a private security firm, Mayor Antonio Villaraigosa and others involved in the selection must act quickly or face the less palatable option of putting an interim leader in place -- something Villaraigosa said during a news conference that he'd rather not do.

Several people from inside the LAPD are obvious candidates for the job -- Bratton's three assistant chiefs, Jim McDonnell, Earl Paysinger and Sharon Papa. Another likely internal candidate is Deputy Chief Charlie Beck, a highly regarded veteran who has risen rapidly under Bratton and oversees the department's detectives. They all declined to comment, saying they were still trying to digest the news of Bratton's departure and that it would be inappropriate to look ahead while he was still chief.

George Gascon, a former LAPD deputy chief who left several years ago to take over a smaller department in Arizona and recently was tapped as the new chief in San Francisco, has also been mentioned by LAPD-watchers as a possible replacement from the outside. Gascon could not be reached for comment, but Bratton went out of his way to tamp down that speculation, saying he believed it was unlikely Gascon would apply because of his recent hiring.

With many in the department believing that Bratton would finish, or at least come close to finishing, the three years left in his current term, his announcement was certain to accelerate what had been up to now the early, quiet stages of jockeying among those with ambitions of assuming the LAPD's top post.

During a private afternoon meeting with his command staff at the department's training academy, Bratton emphasized that he wanted his replacement to come from the team of 12 deputy and assistant chiefs he relies on to run the day-to-day operations of the department. "He told us, 'I strongly believe that the next LAPD chief is sitting here in this room,' " said one high-ranking official who asked not to be named because the meeting was not open to the public.

Villaraigosa, for his part, refused to tip his hand on whether he would prefer to see finalists for the job come from within or outside the department.

"We're going to give a very, very extensive opportunity to people within the department and outside. I certainly believe there are people inside the department who are capable, who are committed to reform, who have the wherewithal to lead this department, but I'm not going to in any way limit opportunities," he told reporters.

Members of the Police Commission, the civilian board that oversees the LAPD, took an equally noncommittal stance. Under the terms of the City Charter, the city's personnel department must conduct a search for candidates and pass along at least six names to the Police Commission. The commission, in turn, selects and ranks three finalists and, from those, Villaraigosa picks a new chief or rejects them all and demands more choices. The City Council must ratify the mayor's ultimate choice.

For all the talk of his replacement, there was plenty of attention paid to Bratton as well Wednesday.

"With Chief Bratton at the helm, the Los Angeles Police Department transformed itself into a beacon of progress and professionalism, a department seen as a partner, not an adversary, no longer bound by the misdeeds of the past," Villaraigosa said.

Many others echoed the mayor with praise for Bratton, who has dramatically reshaped the LAPD and pushed down crime rates since taking over in 2002.

"He is leaving at the top of his game, with a long list of accomplishments and on his own terms," Police Commissioner Alan Skobin said. "There isn't anyone who can argue credibly that this department isn't in better shape than it was before he arrived. That being said, there's still much work to be done and I am saddened that he won't be around to do it."

Ramona Ripston, executive director of the ACLU of Southern California, expressed disappointment that Bratton had clashed with her group over the issue of racial profiling, but called his resignation "a great loss for the city of Los Angeles. He believes in community policing, and he restored the confidence of the community in the LAPD. I watched three prior police chiefs run the LAPD, and the reality is that progress was not made until Chief Bratton became chief and imposed his will and values on the department."

For his part, Bratton expressed satisfaction that he had accomplished what he set out to do when he arrived nearly seven years ago. But, his voice catching at times and his eyes watering, he said it was not easy to take off a badge he said he has worn with pride.

"There is never a good time to leave, but there is a right time," he said. "It is the right time."

Bratton had kept his decision to resign very much to himself. As the chief flew back to Los Angeles on Tuesday night after finalizing the terms of his new job in New York, aides to Villaraigosa said the mayor knew nothing of the planned departure.

Bratton informed the mayor of his decision to resign late Tuesday night, shortly after Villaraigosa returned on a flight from Iceland, where he had been vacationing for the last week. Villaraigosa did not provide any details of the conversation Wednesday, but a source familiar with the situation said Bratton was steadfast in his decision to leave.

A week ago, Bratton scheduled a meeting with the mayor for Wednesday, but did not specify what he wanted to discuss.

Likewise, members of the Police Commission, as well as the deputy and assistant chiefs whom Bratton entrusts with the day-to-day operation of the department, knew nothing of Bratton's plans until the chief informed them Wednesday morning.

"I'm really in shock. It's a great loss for the city and the LAPD," John Mack, the head of the commission, said in an interview after he received word from the chief. "Bratton has done an amazing job of turning around this department in the aftermath of a tortured history and really created a new department for the 21st century."

Bratton, 61, will become the chief executive of a newly formed firm that will consult with governments, primarily in developing and conflict-ridden countries, to help build and improve police forces.

He was lured to the job by Michael Cherkasky, the former federal monitor who until last month oversaw the LAPD as part of a consent decree forced on the department after the Rampart corruption scandal. Cherkasky, a longtime associate of Bratton's, runs the parent company under which Bratton's firm will operate. Bratton said he would relocate to New York City to run the firm.

It was in Los Angeles that Bratton cemented his reputation as one of the country's leading law enforcement minds.

As he did during a short stint as head of the New York Police Department, Bratton implemented a crime-fighting strategy in Los Angeles built around an obsessive focus on crime data and a computer-mapping system that is used to identify specific areas of the city that require more policing.

That approach, along with a management style that placed considerable authority in the hands of his field commanders, produced results: Crime rates have fallen steadily each year since Bratton's takeover.

Bratton also has pushed hard to close long-running chasms of distrust and antagonism between the department and the city's black and Latino communities that stretched back to the Watts riots and through to the Rodney G. King beating, the 1992 riots, the Rampart scandal and the May Day 2007 melee in MacArthur Park.

He reached out to vocal critics of the department, overhauled how recruits were trained, and moved the department away somewhat from a pervasive, decades-old mentality that officers had to remain separate and isolated from the communities they serve.

Bratton's departure comes as the city's precarious finances threaten to not only stem Los Angeles' ambitious police-hiring program, but also to trigger work furloughs for patrol officers and other police personnel. The LAPD must trim $130 million in spending, a consequence of the cuts imposed by the City Council and mayor to close Los Angeles' $530-million budget shortfall. Police administration officials are considering imposing mandatory furloughs starting in October.

It remains to be seen whether the changes made over the last 6 1/2 years under Bratton have taken deep enough root to outlast the man who oversaw them. In recent interviews with The Times, Bratton has said that he believed the department was prepared for his departure. "If I left tomorrow," he said in December, "this would continue after I'm gone."

He has long brushed aside frequent rumors about his leaving the LAPD for other jobs. British tabloids have often breathlessly announced that he was a front-runner to take over Scotland Yard. And during the recent presidential campaign, he was seen as a strong candidate for a top federal law enforcement job, such as at the Department of Homeland Security. When asked by The Times last month whether his decision to place his Los Feliz home up for sale was a portent of some brewing decision to leave, he said he had no such plans.

Regardless, the termination of the consent decree last month seemed to signal a major turning point for Bratton and his outlook on his tenure at the LAPD. With the department now free of what he believed was the heavy stigma of federal oversight, there appeared to be no big new challenge for Bratton to focus on.

"It has been a remarkable seven years," he said. "But it is time to move on."