WASHINGTON (Reuters) - The Cyrus Cylinder of ancient Persia, a landmark in religious freedom and a potent symbol of Iranian national identity, will begin its first U.S. tour with an exhibit that opens Saturday in Washington.

The barrel-shaped clay artifact, 2,500 years old and only 9 inches long, has been described as the first declaration of human rights and an influence on leaders from Alexander the Great to Thomas Jefferson.

It has been celebrated by the shah of Iran and that country's post-revolutionary leaders, as well as by the ancient Hebrews and the founders of modern Israel.

"The Cyrus Cylinder and Ancient Persia" exhibit, opening at the Arthur M. Sackler Gallery, "is about understanding the way Iranians see themselves in the world, and that's obviously important at the moment," said Neil MacGregor, director of the British Museum, which loaned the piece.

The artifact bears inscriptions proclaiming the victory of Persian ruler Cyrus over Babylon in the sixth century B.C. It records the Persian emperor's restoration of shrines dedicated to different gods and his intention to allow freedom of worship to people displaced by defeated ruler Nabonidus.

Such declarations of religious tolerance were not uncommon at the time, but Cyrus' was unique in its nature and scope.

"The cylinder has acquired a special resonance, and is valued by people all around the world as a symbol of tolerance and respect for different peoples and different faiths," the British Museum says on its website.

More than a million people flocked to see the cylinder when the museum lent it to Iran in 2010, in one of the most-viewed exhibits in the country's history.

Cyrus' proclamation was written in spiky Babylonian cuneiform on the soft clay cylinder, which was buried in Babylon, now in modern-day Iraq. It was dug up in 1879 and has since been in the British Museum.

The exhibit runs through April 28 at the Sackler, which is part of the Smithsonian Institution, then travels to Houston, New York, San Francisco and Los Angeles.

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Cyrus Cylinder, ancient decree of religious freedom , starts U.S. tour