At St. Patrick's Cathedral, we have not only employed ushers (security guards), but also what appear to be off-duty cops (who get hired in NYC largely because they can carry concealed weapons). They stand a short distance to either side of the celebrant (including the Rector as well as the Cardinal) as he distributes communion (at least on Sundays, I think it's just security guards during the week).
I've also seen uniformed police come in and arrest someone (immediately) upon his starting shouting during a mass celebrated by the Cardinal, so I think they are usually around , at least off stage, when the Cardinal celebrates. I've twice seen Emergency Services Unit (NYPD SWAT) with shoulder arms and helmets outside the Cathedral (once during Gay Pride Day last Sunday and one other time).
Almost always, there are uniformed police on the steps of the Cathedral when I go in to Mass there.