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.

Schedule changed

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.

The Vatican did not publicly explain why Italian Cardinal Camillo Ruini gave Mass in the pope's place.

However, a Vatican official who spoke on condition of anonymity said that while the pope's doctors have said that the knee is improving, they have been adamant that he take it easy. John Paul at first balked at the idea of not celebrating the Mass, the official said.

Even seated, the pope seemed taxed by the ceremony. Many in the crowd shivered and tried to keep warm by slipping on gloves or pulling up hoods while the blustery wind buffeted the hatless and tired-looking pontiff and whipped through his thin, silver-white hair.

He did kneel at one point for several minutes, and, helped by aides, stood at the ceremony's end to give his blessing. When it was time to go, the two aides flanking him appeared uncertain about how to help him out of his chair. His Polish secretary, Bishop Stanislaw Dziwisz, rushed out of the basilica to lend a hand.

"Long live the pope!", a man shouted out in Italian during a silent moment before the pope left the altar. Cheers rose up from the crowd.

After the ceremony, the pope was driven through the piazza in a white, open-top vehicle as the crowd waved palm fronds and olive branches.

Health problems

Symptoms of Parkinson's disease, including slurred speech and hand tremors, have made public appearances a test of John Paul's stamina. On Sunday, his words were often difficult to understand.

The Palm Sunday ceremony commemorates Jesus' entry into Jerusalem in the days leading to his crucifixion and opens Holy Week, the most important liturgical period for the Roman Catholic church. On past Palm Sundays, the pope has celebrated the Mass and led a procession to bless palm branches held by the faithful.

John Paul faces a heavy schedule of Holy Week ceremonies, including Good Friday prayers at the Colosseum, a Saturday night Easter Vigil Mass in St. Peter's Basilica and Easter Sunday Mass in St. Peter's Square.

The pope is also insisting on continuing to travel widely, planning visits in May to the Italian island of Ischia and Bulgaria and a trip to Canada, Mexico and Guatemala in July. At Sunday's ceremony, he told young people from Canada that he would see them in Toronto in July.