"In September 2008, state and local law enforcement agencies employed more than 1.1 million persons on a full-time basis, including about 765,000 sworn personnel (defined as those with general arrest powers). Agencies also employed approximately 100,000 part-time employees, including 44,000 sworn officers. These findings come from the 2008 Bureau of Justice Statistics' (BJS) Census of State and Local Law Enforcement Agencies (CSLLEA), the fifth such census to be conducted since the quadrennial series began in 1992. From 2004 to 2008, state and local agencies added a net total of about 33,000 full-time sworn personnel. This was about 9,500 more than agencies added from 2000 to 2004, reversing a trend of declining growth observed in prior 4-year comparisons based on the CSLLEA. Local police departments added the most officers, about 14,000. Sheriffs' offices and special jurisdiction agencies added about 8,000 officers each. From 2004 to 2008, the number of full-time sworn personnel per 100,000 U.S. residents increased from 250 to 251."
Bureau of Justice Statistics: http://www.bjs.gov/