NEW : Police Corp . says crowds were vocal but orderly . No arrests as opposing groups demonstrate over racial issues . Police talk about a transit officer who shot and killed an unarmed black man . Officer was found guilty of involuntary manslaughter in the shooting of Officer Grant .
NEW : Police Lt. says crowds were vocal but orderly . No arrests as opposing groups demonstrate over transit killing . Groups demonstrate about a transit officer who shot and killed an unarmed black man . Officer was found guilty of involuntary manslaughter in the shooting of Oscar Grant .