Pope John Paul II has said he hopes the return of a precious Russian icon to the Russian Orthodox Church will lead to reconciliation and understanding.
The bejewelled icon is a powerful symbol for Russian believers
The icon of the virgin of Kazan was symbolically handed over by the Pope to a senior cardinal during a rare Vatican performance of the liturgy in Russian.
Cardinal Walter Kasper will carry the icon to Moscow where he will present it to Patriarch Alexy II on Saturday.
The icon is a copy of one of the most venerated religious images in Russia.
The original was stolen 100 years ago from a church in the city of Kazan, 600km (372 miles) east of Moscow, and may have been destroyed.
The BBC's David Willey in Rome says this particular copy, encrusted with silver and precious stones, has been in the Pope's possession for the past 11 years. It was bought for $1m (£556,000) from a dealer in the United States and presented to the Pope by a group of American Catholics.
Until today, the Pope had kept the painting hanging above his desk in his private apartments. The Pope showed it to Russian President Vladimir Putin when the Russian leader visited the Vatican last year.
On Thursday, the icon will be displayed for the first - and last - time inside Saint Peter's Basilica for veneration by the faithful.
During the ceremony, the Pope said he hoped the "ancient image of the Mother of the Lord" would speak to Patriarch Alexy II and the Orthodox Church of the Pope's affection for them.
"May it speak to him of the desire and the firm will of the Pope of Rome to progress together with them on the path of mutual understanding and reconciliation, to speed the day of full unity of the faithful, for which the Lord Jesus ardently prayed," he said.
Our correspondent says relations between the Catholic and Orthodox churches, which separated 1,000 years ago, have been tense in recent years.
The Russian Church leader has consistently opposed a Papal visit to Russia on the grounds that the Catholic Church has pursued an expansionist policy inside Russian territories trying to poach converts - a claim the Vatican denies.