At Chak Chak, some 600 kilometres southeast of the capital Tehran, believers gathered to remember Nikbanou, a heroine of the faith who according to tradition took shelter in the mountain and prayed for help. Miraculously, the mountain was said to have opened up and given protection to the princess, the youngest daughter of the last king of the Persian Sassanian empire.
A spring slowly drips from the ceiling of the shrine, built into a cliff-side cave, giving the site its name, which means "drip drip" in Persian.
"We've all gathered here to remember Nikbanou and celebrate our deep Persian roots," said priest Ardeshir Khorshidian who was dressed in white to symbolise purity.
Many pilgrims stay overnight at pavilions set up at the base of the mountain. Families sitting on rugs had picnics while children played.
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Dariush Tirandazi, an Iranian-American Zoroastrian, brought his 22-year-old daughter Nadia so she could experience the faith in the land where it was born.
"She wanted to learn and this was our chance to come here and show her the temple, to give her the chance to be with
Zoroastrianism is a monotheistic religion predating Christianity and Islam founded some 3,800 years ago by Zoroaster. It was the dominant religion in Persia before the Arab conquest.
It stresses good deeds, and fire plays a central role in worship as a symbol of truth and the spirit of God.
Thousands of Zoroastrians, like many other Iranians, immigrated abroad under former president Mahmoud Ahmadinejad because of social restrictions and a worsening economy. But now a dozen have returned to live in Iran this past year, the government said.