Cardinal Francis George said mass Sunday morning at Holy Name Cathedral.
According to the Bible, Palm Sunday marks the day Jesus rode into Jerusalem and was greeted by cheering crowds bearing palm fronds.
In Jerusalem, hundreds of Christians marched Sunday from the Mount of Olives, retracing the steps Jesus took 2,000 years ago.
Pilgrims and tourists packed into St. Peter's Square in Rome to see Pope Benedict. In his sermon, he said that man's achievements have increased the possibilities for doing evil as well as good.
Orthodox Greeks are also celebrating Palm Sunday. At a church in Athens, the faithful lit candles and the priest burned incense while prayers were offered up.