The head of Russia's Orthodox Church wields considerable political weight
The head of the Russian Orthodox Church, Patriarch Kirill, has told worshippers in Ukraine their breakaway Church must reunite with Moscow.
After leading prayers at the Monastery of the Caves in Kiev, he appealed to them to "return to the father's house".
The Ukrainian Orthodox Church of the Kiev Patriarchate broke away from the Moscow Patriarchate in the early 1990s.
Thousands marched through the Ukrainian capital on Tuesday to protest against the patriarch's visit.
Demonstrators carried placards denouncing what they see as Russian interference in their country, and in support of the Kiev Patriarchate.
The protest coincided with a service at the Pechersk Lavra marking the 1,021st anniversary of the conversion to Christianity of Kievan Rus, the ancient state that gave rise to modern-day Ukraine and Russia.
During the ceremony, Patriarch Kirill called on the Kiev Patriarchate to reunite with the Moscow Patriarchate, which considers it schismatic.
"Not all our brothers of the same faith share this holiday today," he said. "Some find themselves outside the Church's saving fence, outside its precious unity."
"The aspiration to brotherly unity has not melted among Orthodox Ukrainian people. I saw it myself today when I was looking at you all," he added.
The Kiev Patriarchate broke away from the Moscow Patriarchate in 1991, when Ukraine gained its independence.
It has sought the status of a legitimate, independent national Church, not answering to Moscow, from the most prominent figure in the Orthodox world, Patriarch Bartholomew of Constantinople. But he has provided no clear response.
Some Ukrainians have denounced what they see as Russian interference
Its efforts are also strongly backed by Ukrainian President Viktor Yushchenko, who believes a recognised independent church is a key part of strengthening Ukraine's national identity.
However, BBC Russian affairs analyst Steven Eke says the Russian Orthodox Church wields considerable political weight, and plays a role in the Kremlin's policies aimed at strengthening the Russian state and its influence abroad.
This is what makes Patriarch Kirill's visit to Ukraine so divisive, our correspondent says.
Nationalist groups, many Ukrainian-speakers and the congregations of the Kiev Patriarchate see him less as a religious pastor, and more as a political activist seeking to boost the Kremlin's influence in their land, he adds.