From the NYPD web page.

http://www.nyc.gov/html/nypd/html/administration/mission.shtml

The MISSION of the New York City Police Department is to enhance the quality of life in our City by working in partnership with the community and in accordance with constitutional rights to enforce the laws, preserve the peace, reduce fear, and provide for a safe environment.

IN PARTNERSHIP WITH THE COMMUNITY, WE PLEDGE TO:

Protect the lives and property of our fellow citizens and impartially enforce the law.
Fight crime both by preventing it and by agressively pursuing violators of the law.
Maintain a higher standard of integrity than is generally expected of others because so much is expected of us.
Value human life, respect the dignity of each individual and render our services with courtesy and civility.

There is no "motto" for the NYPD.