Atlantic City Police arrested a city man Sunday after he allegedly brandished a knife at an off-duty officer who stopped running a charity race to break up a violent incident on the beach, the department said.Lt. James Herbert was participating in the Seedorf/McMeekin Memorial 5K Run on the Boardwalk at about 10:10 a.m. when he noticed a man on the beach who had a woman in a headlock, pulling her toward the underside of the Boardwalk, police said Monday in a news release. Herbert immediately intervened and stopped the man, Xontae Glass, 27.