Eastern-rite Catholics, who follow Orthodox ritual but recognise the Pope as their leader, have long wanted to move from their stronghold in western Ukraine to Kiev, capital of the young Ukrainian state and a bulwark of Orthodox tradition.
Orthodox objections to the move underscored the difficulties of Pope Benedict, who was completing his first international trip to Germany today, to repair ties between the two Christian churches, split since the Great Schism of 1054.
Cardinal Lubomyr Husar told about 3,000 faithful it was natural for Catholics to have their top clergy in Kiev, where Christianity took hold late in the 10th century.
''Thanks to monks and missionaries, Christianity made its way from here -- in Kiev -- throughout the Slav world. Kiev earned fame as the cradle of Christianity in the Slav east,'' Husar said outside a vast cathedral under construction.
''But we allowed the church that was established in this holy place to be divided. And we ask ourselves: is there a way to restore that initial unity to bring confrontation to an end?'' Ukraine's Orthodox faith makes up a majority of believers in the ex-Soviet state, but is itself divided. Only the largest group, the local branch of the Russian Orthodox church, objects to the move and accuses Catholics of poaching parishes.
Benedict's predecessor John Paul, a Pole who opposed communist rule in eastern Europe, made the first visits by a pontiff to mainly Orthodox states, including Ukraine in 2001.
Opposition by Patriarch Alexiy II thwarted his desire to visit Russia. Relations with the Orthodox Church are difficult.
As the mass proceeded, riot police kept away hundreds of Orthodox activists who, backed by extreme leftist politicians, staged their own impromptu service from the back of a truck. But suggestions the protests could turn violent proved unfounded.
''Traitors, fascists!'' a middle-aged woman shrieked at a Catholic priest, who smiled and pointed skywards.
''Wherever there were Catholics in our country, blood was spilled,'' shouted a man waving a large red banner.
In the run-up to today's service, Alexiy said the move violated church canons and was incompatible with Ukraine's Orthodox tradition.
The eastern-rite, or Greek Catholic, church was formed at the end of the 16th century in western Ukraine, ruled for centuries, in turns, by Poland and the Austrian empire.
Banned by Josef Stalin in 1946, its clergy were imprisoned or exiled and its churches handed to Orthodox parishes.
Congregations gathered secretly in forests or basements.
With the ban lifted shortly before the 1991 collapse of Soviet rule, Catholics began demanding the return of churches from Orthodox parishes, sparking occasionally violent clashes between elderly believers through the 1990s.
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