TEHRAN, April 21 (Reuters) - About 100 Iranians protested in Tehran against a new dam on Saturday, saying it threatened to damage archaeological sites in the south of the country dating to the first Persian empire almost 2,600 years ago.

The protesters carried banners with symbols from Iran's pre-Islamic Zoroastrian religion and demanded the resignation of Esfandiar Rahim Mashai, head of the state culture and heritage organisation. He has rejected such criticism in the past.

"Mashai, you are the bloodthirsty Genghis, you are Alexander," they chanted, referring to Mongol ruler Genghis Khan and Alexander the Great who invaded and sacked Persia at different times in history.

The student-led protest came two days after authorities started flooding the dam after months of controversy. It lies 9 km (5.5 miles) from Pasargadae, the first capital of the Persian empire where its founder Cyrus the Great was buried.

Critics say the dam will increase humidity and possibly damage structures such as his tomb. They also say it would submerge the nearby Bolaghi Gorge, through which one of the Persian empire's most important roads ran.

"The flooding of Sivand will definitely have a destroying effect on the historical remains and memories of this region," said a statement issued by the protesters.

The famed ruins of Persepolis, sacked by Alexander in 330 BC, are in the same area but further away from the dam.

Mashai was not available for comment on Saturday's protest.

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