The Russian Orthodox Church has portrayed it as the biggest split in Christianity for 1,000 years.
Ukraine has ended hundreds of years of religious dominance by the Russian Orthodox Church.
Thousands gathered in front of the Orthodox Saint Sophia Cathedral in Kiev on Saturday as religious leaders met there to formalise the split and enlarge an already existing Ukrainian Orthodox church.
The head of the world's Orthodox Christians in Istanbul had already ruled that the split should go ahead in October.
That infuriated the Russian church - it denounced it as a schism and severed ties with Istanbul.
It ended 332 years of Russia, the world's largest Orthodox church, acting as Ukraine's spiritual guardian.
But the dispute is as much about politics as religious doctrine.
Russia annexed Crimea from Ukraine in 2014 to worldwide condemnation.