The British Museum has lent one of the Elgin Marbles for the first time.
A headless depiction of the river god Ilissos has been sent to Russia to go on display in St Petersburg's State Hermitage Museum until mid-January.
It is one of a number of relics acquired by Lord Elgin in Athens in the early 19th Century, now known collectively as the Elgin Marbles.
Ownership of the artefacts, once part of the 2,500-year-old Parthenon temple, is disputed by Greece.
It maintains that Lord Elgin removed them illegally while the country was under Turkish occupation as part of the Ottoman Empire. The items have remained in the British Museum ever since.
Neil MacGregor, director of the British Museum, said it was a "very big moment".
He told BBC Radio 4's Today programme: "This is the first time ever that the people of Russia have been able to see this great moment of European art and European thought."
Mr MacGregor added that he hoped the Greek government would be "delighted".
"I hope that they'll be very pleased that a huge new public can engage with the great achievements of ancient Greece. People who will never be able to come to Athens or to London will now here in Russia understand something of the great achievements of Greek civilisation."
Asked whether the marbles would be loaned to Greece if it promised to give them back, he said the museum was willing to lend anything in the collection provided it was fit for travel and if it was going a place where it would be safe and from where it would be returned.
The Greek government, he added, had to date not asked to borrow them.
Analysis - Trevor Timpson, BBC News
The statue of Ilissos, with its beautifully carved drapery suggesting river water, is one of the sculptures which adorned the triangular ends (the pediments) of the Parthenon.
The British Museum was willing to send it to the Hermitage for its 250th anniversary, but sent nothing to the new Acropolis museum in Athens, where places are reserved for Ilissos and all the other missing sculptures.
British Museum director Neil MacGregor must have been on tenterhooks when it disappeared from show, in case anyone asked what "display" it was being prepared for.
Sending it in secret to Russia was a nifty piece of footwork. Equally nifty would be if the Greeks could persuade Mr Putin to "return" it - but to Athens, not London.
Mr MacGregor says he is willing to start a dialogue with the Greeks about loans of this kind - and the idea of a permanent loan (of the whole Elgin collection) has been suggested in the past.
But for many Greeks, the idea of borrowing back what they regard as their own property would be painful.
BBC Monitoring says the loan of the Elgin Marbles did not get front page coverage in major Greek newspapers on Friday, but it was reported widely.
Most simply related the story, relying on British media reports for detail, and recalled Greece's long campaign to have the sculptures returned.
Kathimerini and Ta Nea both saw the loan as act of defiance, given Greek wishes.
In a blog about the loan, Mr MacGregor said the British Museum was a "museum of the world, for the world".
He noted that the British Museum had opened its doors in 1759 and the Hermitage just five years later - making them "almost twins... the first great museums of the European Enlightenment".
The British Museum was today "the most generous lender in the world", he said, "making a reality of the Enlightenment ideal that the greatest things in the world should be seen and studied, shared and enjoyed by as many people in as many countries as possible".
"The trustees have always believed that such loans must continue between museums in spite of political disagreements between governments."
The Parthenon Marbles
- Friezes and pediment figures which decorated the Parthenon temple in Athens, built 447-432 BC
- Many were removed by agents of the British diplomat Lord Elgin in the early 19th Century, and eventually sold to the British Museum
- Most of the surviving sculptures are roughly equally divided between London and Athens
- The new Acropolis Museum opened in Athens in 2009. It is designed to display all the surviving sculptures, in their original layout
- Celebrities previously involved in the campaign for their return include the late actress and former Greek Culture Minister Melina Mercouri