 Some Shia have complained of mistreatment in countries dominated by Sunnis, especially in Saudi Arabia, while some Sunnis have complained of discrimination in the Twelver-dominated states of Iraq and Iran.Some tension developed between Sunnis and Shia as a result of clashes over Iranian pilgrims and Saudi police at the hajj.Millions of Saudi adhere to the school of Salafism which is a branch of Sunni Islam.