Ukraine's Eastern-rite Catholics have moved the headquarters of their church to the capital, Kiev, amid protests by some 300 mainly Orthodox believers.
Protesters tried to prevent believers from attending the service
The head of the Greek Catholic Church, Cardinal Lubomyr Husar, held a Mass for some 1,000 believers to mark the move from the western city of Lviv.
Eastern-rite Catholics follow Orthodox ritual but bear allegiance to the Pope.
The move could strain ties between the Vatican and Russia's Orthodox Church, which has huge influence in Ukraine.
The head of the Russian Orthodox Church, Patriarch Alexy II, had earlier described it as an "unfriendly" act.
Bells rang as Cardinal Husar conducted the service outside a church being built for followers of Ukraine's Greek Catholic (or Uniate) Church.
Cardinal Lubomyr Husar appealed for reconciliation
"We don't threaten anybody, we don't want to threaten any Orthodox Church," the cardinal said.
"The headquarters of any religious community should be in the capital," he added.
The mass took place amid tight security, as several hundred Orthodox protesters gathered near the site, denouncing the move.
"Orthodox or death!" they chanted, accusing the Vatican of proselytising.
"You can kill us but you cannot take our faith from us!" demonstrators shouted.
The service was attended by officials from the Ukrainian Orthodox Church (Kiev Patriarchate), which is not recognised by the Russian Orthodox Church.
The Uniate Church claims five to six million adherents - mainly in western Ukraine - plus a further 1.5m among ethnic Ukrainians living in Russia.
The Church was originally established in 1596, when most of today's Ukraine was part of Catholic Poland.
The "Union" was controversial from the start. Orthodox Christians saw it as an annexation of part of their own church by Rome.
With the expansion of the Orthodox Russian empire into Ukraine, in the 17th and 18th Centuries, the Uniate Church was banned.
It survived mainly in those parts of the old Polish state which were incorporated into the Catholic Austrian Empire in 1772.
These areas, eventually annexed by the Soviet Union during World War II, now form part of western Ukraine.
In deciding to move its headquarters to Kiev, the Uniate Church is indicating that it sees itself as a pan-Ukrainian church - rather than a regional peculiarity, the BBC's European affairs analyst Jan Repa says.