VATICAN CITY - Security has been heightened in the last few months at the Vatican over concerns that the heart of Roman Catholicism could be target of Islamic extremists.

Italian police officers patrolled the crowd, and Vatican plainclothes security personnel kept a close eye on participants as Pope John Paul II celebrated Palm Sunday Mass.

John Paul was in the company of thousands of young people who joyously waved fronds and olive branches and cheered encouragement as he began a heavy schedule of Holy Week ceremonies.

The Vatican described the gathering of more than 40,000 faithful in front of the altar on the steps of St. Peter's Basilica as an "antidote" to the fears of terrorism and war around the globe.

Still, the atmosphere, under a hazy sun which burned away morning fog, was relaxed. After blessing participants at the end of the two-hour ceremony, the pope spoke briefly with a young girl who broke away from the crowd and headed for his chair.

Then John Paul was driven around the square in an open-sided "popemobile," stopping to kiss babies held out near him. In the afternoon, he made an impromptu appearance from his window overlooking St. Peter's Square, where hundreds of youths were still gathered.

The coming week will see John Paul preside at Holy Thursday services at the Vatican, a Good Friday procession at the Colosseum and Saturday night and Sunday morning Easter ceremonies.

John Paul, who turns 84 next month and who has Parkinson's disease, looked wan and weak as he clutched a braided palm at the start of the ceremony. But he later seemed to gain strength, nodding in pleasure at cheers and applause from the crowd.

The pontiff urged young people to prepare for World Youth Day in Cologne, Germany in the summer of 2005. But when a Berlin university student, in a TV hookup from that city, invited the pope to join them in Cologne, he didn't answer.

Cologne's Cardinal Joachim Meisner has said the pope will go "if God gives him the strength."

John Paul has been the star at Youth Day events held every two years since the mid-1980s, from gatherings in Buenos Aires, Argentina and Denver, to Toronto and Manila, the Philippines.

On the years that World Youth Day doesn't take place, kids and teens from around the world come to celebrate in the Vatican on Palm Sunday.

Despite his health problems, the Vatican has never ruled out further travel, and a Vatican advance team surveyed Switzerland a few weeks ago for a possible trip there in June.

The pontiff slowly read his homily, in which he recalled that Palm Sunday marked Jesus' triumphant entry into Jerusalem, only days before his crucifixion. The crowd greeting Jesus was "so fickle that in a few days it went from joyous enthusiasm to homicidal scorn," John Paul said.

The Vatican newspaper L'Osservatore Romano published photographs of terror bombings in Madrid, Middle East violence and the Iraq war, and described the gathering as a "powerful antidote" to fear.

"The most insidious enemy of young people is called fear," the Vatican paper said. The pontiff, it said, invites youths to "ally themselves on Christ's side, and not with the sowers of hate, the mercenaries of terror, the prophets of death."

John Paul told the crowd: "Certainly the message that the Cross communicates isn't easy to understand in our era, in which material well-being and convenience are proposed and sought after as priority values."