Thousands of people in Ukraine took part on July 27 in a Moscow-backed religious march through central Kyiv to mark the 1,030th anniversary of country's adoption of Christianity.
The participants in the procession were supporters of the branch of the Ukrainian Orthodox Church backed by Moscow.
According to police, some 20,000 people took part in the event, while organizers said 250,000 people attended.
Some 5,000 police officers were deployed to ensure order amid fears of possible incidents with Ukrainian nationalists.
President Petro Poroshenko, who met on July 27 with a delegation from the Ecumenical Patriarch Bartholomew I of Constantinople, is expected to attend a rival religious procession backed by the breakaway Ukrainian Orthodox Church.
Kyiv used to be the capital of the pagan Slavic state of Kievan Rus, which was converted to Christianity in 988 by Prince Vladimir.
The Ukrainian Orthodox Church declared autonomy from the Russian Orthodox Church after the country gained independence in 1991.
The Kyiv-based Ukrainian Orthodox Church supported a popular revolt that ousted Kremlin-backed President Viktor Yanukovych in 2014.
Russia's annexation of Ukraine's Crimea region in March 2014 and the ensuing conflict between Kyiv and Russia-backed separatists in eastern Ukraine have stepped up religious tensions as well, with Patriarch Filaret of the Kyiv-based Ukrainian Orthodox Church famously saying Russian President Vladimir Putin was "possessed by Satan."
The July 27 march sparked criticism in Ukraine, where many consider members of the Moscow-backed church as agents of the Kremlin.
Meanwhile, Russian Orthodox Patriarch Kirill warned against attempts to stir discord between the Ukrainian Orthodox Church and Moscow.
"Attempts to artificially tear the Ukrainian Orthodox Church away from the Moscow Patriarchate can lead to a catastrophe," he said.