The United States has condemned violence in Iran where four to eight people died in clashes between opposition demonstrators and police on Sunday.

"We strongly condemn the violent and unjust suppression of civilians in Iran seeking to exercise their universal rights," Mike Hammer, a National Security Council spokesman, said in a statement.

Iranian police reported Sunday that 300 protestors were arrested. The authorities have denied that the victims were killed by police.

The clashes on Sunday came as the country marks Ashura, a 10-day period of religious ceremonies which commemorate the death of Imam Hussein, a grandson of the Prophet Mohammed who was killed and beheaded in the Battle of Karbala in 680 AD by the armies of the Sunni caliph Yazid.