Discussion of reforms, which includes the possible switchover from Church Slavonic to modern Russian in church services, intensified after the death of Patriarch Alexy II early this month.
Alexy, who led the revival of Orthodoxy in the country after the demise of communism, died of heart failure at the age of 79.
"If a reform destroys values, it is called heresy. I am strongly opposed to any reform," Kirill told reporters in the patriarch's residency in Moscow ahead of Orthodox Christmas celebrated on January 7.
He said other candidates to the patriarchal post were not "reformists" either.
Kirill, Metropolitan of Smolensk and Kaliningrad, was chosen by the Holy Synod as interim patriarch until a larger ruling body gathers on January 27-29 to elect Alexy II's successor. The new patriarch will have to address relations with the state and the Catholic Church.
Kirill, 62, who heads the Church's department for external relations, has led dialogue with the Catholic Church and has worked to rebuild ties with other churches across the world. He is also well known to the public in Russia through his weekly television program Pastor's Word.
He said reforms that damage tradition were dangerous. "The Church is a conservative entity as it is to preserve the apostolic faith through generations," Kirill said.
He said the most striking examples of church reform in Russia - that of the 17th century to establish uniformity between Greek and Russian church practices and the Renovated Church reform movement in the 1920s - only led to splits among clergy and believers.
Kirill said the Church does undergo changes through natural processes.
The new Russian patriarch is expected to be enthroned on February 1.
