NEW YORK (WABC) -- Christians around the globe celebrated Christmas as the clock struck midnight Sunday, with many heading out for a traditional Midnight Mass. And at St. Patrick's Cathedral and at churches around the country, there was added security amid a call from ISIS to attack target U.S. places of worship.It was a show of force at Manhattan's most famous church, with the level of security visibly increased as the evening progressed. The entire block around St. Pat's became a frozen zone, blocked off to traffic and with even the flow of pedestrians tightly controlled.The NYPD's counter-terrorism unit was out, with heavily-armored officers and the bomb squad posted around the iconic building.There wasn't an empty seat in the house for the 5:30 p.m. Mass, and standing room only was the norm as onlookers pressed up against the glass outside hoping for a sneak peak.The heavy security followed fresh terror warnings Friday , although the FBI said it is not tracking any specific or credible threats. But experts say law enforcement must always be ready."One of the most significant threats facing this country today comes from people who are inspired by terrorist ideologies, but operate independent of a terrorist organization," ABC News contributor John Cohen said.St. Patrick's is a New York institution, with Midnight Mass a New York tradition that draws not only New Yorkers, but tourists from all over the world. And with so many descending on the historic Cathedral, it's just another reason why all eyes are on St. Pat's and New York City.