Russian Patriarch Kirill of Moscow and All Russia has begun an eight-day visit to Ukraine.
He will first visit Odesa, in southern Ukraine, before traveling on to Dnipropetrovsk and Kyiv.
Kirill, who is also expected to meet with Russia-friendly President Viktor Yanukovych, has said he wants to see for himself what he called recent "positive changes" in Ukraine.
Supporters say the Russian patriarch's visit is aimed at strengthening spiritual bonds between the two Slavic peoples and reconciliation within the Orthodox Church in Ukraine, which split in 1991 when Ukraine gained independence.
Critics say it is primarily aimed at boosting Russia's religious and political influence.
Kirill's previous visit in 2009 sparked protests by Ukraine nationalists.
RFE/RL's Ukrainian Service, with agency reports