This is how I'd say it.

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Note added at 1 hr (2006-11-12 18:15:10 GMT)
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comisaría = police office (in the US)

comisario = police chief/chief of police

At least in the US (nowadays), the use of "sheriff" is almost exclusively seen in the context of county-level police forces (and not so much in other law enforcement agencies). For example, in the Fort Lauderdale area (in the state of Florida), the police of the Broward County Sheriff's Office (which covers several cities, including Ft. Lauderdale and Hollywood) are commonly called sheriffs. Thus, depending on the context, the best option might be "chief" :-)

Hope this helps!

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Note added at 8 hrs (2006-11-13 01:38:47 GMT)
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Another option for "comisaría" would be "police DEPARTMENT" (which is more common in the US). Also, the law enforcement officials/officers of the sheriff's offices (in US counties) are often called "deputy sheriffs" with the term "sheriff" being reserved for the chief). But again, the title used in the majority of (city) law enforcement agencies would be "chief (of police)," with "chief" (for short) being the typical way one would address the highest ranking officer in the department.

That said, depending on the agency, another option might be "commander" :-)