US President Barack Obama on Monday condemned Iran's crackdown on protesters, saying "history will be on the side of those who seek justice."

The United States "strongly condemns" the oppression of Iranian citizens, Obama said in his native Hawaii where he is taking vacation with his family.

"The United States joins with the international community in strongly condemning the violent and unjust suppression of innocent Iranian citizens," Obama said.

He urged Iranian authorities to immediately free those "unjustly detained."

"We call for the immediate release of all who have been unjustly detained within Iran," Obama said.

"For months, the Iranian people have sought nothing more than to exercise their rights," Obama said. "Each time they have been met with Iran fist of brutality, even on solemn occasions and holydays. Each time the world watched with deep admiration for the courage and conviction of the Iranian people who are part of Iran's great enduring civilization."

A large number of Iranians took to the street on Sunday across the country to commemorate the Shiite Muslim ritual of Ashura. According to Tehran, 8 people died during clashes between protesters and security forces, over 300 were arrested.