Vatican City ? Plagued by knee pain, Pope John Paul II took the exceptional step of ceding his place at the altar during Palm Sunday Mass, the latest sign of the health problems that are exacting a toll on the once tireless pontiff.

To the surprise of tens of thousands of faithful who turned out on a spring day so cold snowflakes wetted the top of the colonnade around St. Peter’s Square, John Paul did not celebrate the Mass. Instead, he sat in an armchair near the altar on the steps of St. Peter’s Basilica, letting an Italian cardinal take his place.

The pope, who turns 82 in May, read the homily and several prayers during the nearly 2 1/2-hour celebration, but the substitution  a severe break with tradition  spared him a long spell standing behind the altar.

Persistent knee pain blamed on arthrosis, a degenerative joint disease, forced the pope to cancel several public appearances recently and abandon, at least temporarily, his cherished tradition of informal Sunday visits to parish churches in Rome.