ROME — On a recent morning on St. Peter’s Square, alongside the bustling priests and nuns, would-be tour guides and vendors hawking cheap umbrellas, military police officers stood guard with submachine guns. But that is just the beginning.

On Tuesday — the start of what Pope Francis has declared a holy year, or Jubilee of Mercy, that is expected to draw millions of visitors — anyone entering the square will be subjected to airport-style screening, including metal detectors and X-ray machines. Pilgrims and tourists take note: Expect long lines.

In the wake of the Paris attacks, jitters about terrorism have spread far beyond the borders of France. Romans are on edge as well.

Rome already attracts more than 16 million tourists each year, according to its tourism department. But the Jubilee, which is usually held every 25 years, with the occasional exception (the last was in 2000), is expected to draw many more. And some Italians are concerned that the city is not equipped to handle the surge.