NEW YORK (REUTERS) - New York City police commissioner William Bratton will retire in September, Mayor Bill de Blasio announced on Tuesday (Aug 2), capping a four-decade career that saw him become the most well known figure in US policing.

Bratton will step down after two years at the helm of the nation's largest city police force, the second time he has served in that position.

Calling his accomplishments "literally inestimable and extraordinary," de Blasio credited Bratton with pushing the city's crime levels to historic lows and working to repair relations between the department and the communities it protects.

Regarded as a celebrity in the world of law enforcement, Bratton, 68, was credited with sharply cutting crime in New York City during his first spell as commissioner in the 1990s. He gained the moniker "supercop" before leaving the department to work in the private sector.

Bratton, who started his career as an officer in Boston in 1970, has also overseen the police departments in Boston and Los Angeles.

He is known as a strong supporter of the "broken windows" theory, which holds that preventing minor infractions such as vandalism and public drinking can help lower the number of more serious crimes.

In New York, Bratton applied the theory by cracking down on petty crimes while working under then-Mayor Rudy Giuliani.

Supporters have credited his "zero tolerance" policing strategy with helping to contribute to the city's drastic reductions in crime since the 1990s, while critics have assailed the tactics as heavy-handed.

In his first turn as police commissioner, Bratton introduced the CompStat system of real-time crime tracking, which has since been adopted by other police departments.

Bratton had previously said he would not serve past the end of de Blasio's current four-year term, which ends next year.

Bratton's successor will be Chief James O'Neill, the department's top uniformed officer.