Archbishop kneels with the Pope in silent unity.
Archbishop George Carey and Metropolitan Athanasios, representing Ecumenical Patriarch Bartholomew, the spiritual leader of the world's Orthodox Christians, knelt in silent prayer with the Pontiff after helping him to push open the Holy Door of St Paul's Outside the Walls Basilica.
Only one cushion for kneeling was provided - for the Pope - underlining the surprise decision of the Anglican and Orthodox representatives to get down on their knees, too, in a gesture of unity.
After opening the Holy Door of St Peter's Basilica on Christmas Eve to usher in the Vatican's Holy Year, the Pope had opened the special doors of two other basilicas that pilgrims visit during the Holy Year to gain indulgences.
As part of his goal of working toward Christian unity in the new Millennium, the Pope invited representatives of other Christian denominations to join him at the last of Rome's main basilicas, St Paul's.
"Unity! Thank you!" the Pope shouted out with joy in impromptu remarks after reading his homily.
The 79-year-old Pope seemed invigorated by the scene before him.
An American woman, Ms Frances Alguire, greeted him as president of the World Methodist Council.
On his arrival, the Pope crossed the basilica's courtyard with an unusually brisk step. Since hip surgery in 1994, he has experienced difficulty walking.
But later, as he prayed, the tremor in his left hand - a symptom associated with Parkinson's disease - was evident when he gripped his pastoral staff for support.
On Sunday, the Pope said he was looking forward to the ceremony in which he and other Church leaders would symbolically ask each other's pardon for the sins committed against Christian unity over the last Millennium.
More than a score of non-Catholic representatives attended, some of them taking the role of deacons or as readers of religious texts.
"We know we are brothers still divided, but we have placed ourselves with decisive conviction on the path that leads to full unity," the Pope told participants, who included Lutherans and Pentecostals.
"We ask pardon of Christ for all that which in the history of the church prejudiced his design of unity," the Pope said.