VATICAN CITY - The spiritual leader of the world's Orthodox Christians prayed with Pope Benedict in the Sistine Chapel on Saturday and urged Catholics and Orthodox to work together to combat fundamentalism and to promote religious tolerance.

Benedict praised his guest, Patriarch Bartholomew, on the occasion of an Orthodox leader's first service in the chapel, which is famous for its frescoes painted by Michelangelo.

Benedict said Bartholomew's participation in the Vespers service and speech, in the chapel where popes are elected, was a joyous experience of unity, perhaps not perfect, but true and deep.

The two men are eager to bridge a nearly millennium-long schism between the two churches, and see moral and social issues -- including fundamentalism, religious intolerance, abortion, euthanasia and environmental degradation -- as fertile ground for common initiatives.

The split between Roman Catholic and Orthodox churches centres on the primacy of the pope.

Bartholomew was invited to address bishops from around the world attending a meeting at the Vatican this month about the importance of the Bible.

Cardinals and bishops listened attentively as the patriarch spoke about the potential for common initiatives between the world's 250 million Orthodox and more than a billion Catholics.

The Orthodox leader called it more imperative than ever for both sides to provide a "unique perspective -- beyond the social, political or economic -- on the need to eradicate poverty, to provide balance in a global world, to combat fundamentalism or racism and to develop religious tolerance in a world of conflict."

Bartholomew described the invitation to address the pope and bishops in the Sistine Chapel as humbling and inspiring.