Here is a link to the 2004 "Persons Arrested" report from the FBI (.pdf). Personally, I'd rather look at the numbers themselves than consider reports from organizations with questionable motives.
According to the FBI, in 2004:
Of all arrestees for violent crimes, 60.4% were white, 28.5% were black, and the remaining 2.5% were of other races.
For all arrests, 70.5% of arrestees were white, 27.1% were black, and 2.4% were other races.
As a side note, 76.2% of arrestees were male and 82.2% of arrestees for violent crimes were male.