To protect and to serve.
Another View: Many (most?) departments have their own motto and the while the above is popular, it is not a universal answer. For example - the motto of the District of Columbia Metropolitan Police is Justitia Omnibus - 'Justice for All'.
The motto of National Bureau of Investigation - Philippines - is 'Efficient law enforcement in the pursuit of truth and justice.'.
The motto of Mount Dora Police Department is 'First in the Nation'.
The state motto of Tasmania is "Fertility and Faithfulness".
Detroit Police Department's motto is 'Making Detroit a safer place to live, work, and visit.'.
West Lake Fire Department's motto is 'The Desire to Serve'.
Greater Sudbury Police Service's motto is 'Our Community, Our Commitment'.
The motto of Seattle Police Department is 'Service, Pride, Dedication'.
Saanich Police Department's motto is 'Populo Serviendo'.
The motto of Huntington Police Department is 'To Serve and Protect'.
The motto of Indianapolis Police Department is 'Integrity Pride Dedication'.
Victoria Police Department's motto is 'Honour through Service'.
The motto of Orlando Police Department is 'Courage, Pride, Commitment'.
Carolina Police Department's motto is 'Honor, Integrity, Service'.
Philadelphia Police Department was created in 1850.
Crown Point Police Department's motto is 'To serve and Protect'.
The motto of Mount Dora Police Department is 'First in the Nation'.
The motto of San Juan Police Department is 'Proteccion, Seguridad'.
Virginia Tech Police Department's motto is 'Ut Prosim'.