Russia's Patriarch Kirill blasted the Ecumenical Patriarchate of Constantinople as "schismatic" for granting independence to the Ukrainian church, a move that pushed Moscow to cut ties with Orthodoxy's mother church.
"The Constantinople patriarchate identified itself with schismatics," the head of the Russian Orthodox Church Patriarch Kirill told a conference in Moscow.
"Uncanonically, violating all rules, it invaded our jurisdiction and forgave schismatics," he added, referring to a branch of the Ukrainian church it does not recognise.
"By identifying with schismatics, it became a schismatic itself," he said.
On Monday, the Russian church said it did not recognise Constantinople's decision to grant Ukraine the right to form an independent church and broke ties with the leading Orthodox authority.
The move was described as one of the gravest crises in the Church's history.
Russia's spiritual leader defended the decision to break with Constantinople, saying most Orthodox believers supported the rupture.
He called on clerics to "clarify the situation on Ukraine" with church goers.
"For the decision to be supported by the people -- and I feel that the absolute majority of Orthodox believers support this decision -- the clergy must also do its work," he said.
Only then, he said, will his church's position be "very strong and convincing."
"I really hope that this will be the case, but pastoral work to clarify the situation on Ukraine is very important," he said.
Russian state media reported that Hilarion, the bishop in charge of the Russian Orthodox Church's diplomacy, met with Pope Francis in the Vatican on Friday in closed-door talks on Moscow's decision to break with Constantinople.
Last week, the Constantinople Patriarchate overruled its own decision from the 17th century which adjoined Kievan Orthodox churches to Moscow.
It lifted the anathema Moscow imposed on Patriarch Filaret, a cleric who set up a Kiev-based Orthodox church in Ukraine after the fall of the Soviet Union.
Ukraine's parliament on Thursday voted to hand over a landmark Kiev church to the Ecumenical Patriarchate following the decision.
Most of the Orthodox parishes in Ukraine have historically been under the umbrella of the Moscow Patriarchate, and many of these may eventually switch to the new independent Ukrainian Orthodox Church, despite Moscow's warnings.