By Parisa Hafezi, Ahmed Elimam and Eman Abouhassira DUBAI, July 4 (Reuters) - Mourners thronged a vast prayer complex in Tehran on Saturday as the week-long funeral ceremonies of Iranian Supreme Leader Ayatollah Ali Khamenei began with the national anthem, religious eulogies and readings from the Koran. Iran is staging mass funeral processions for Khamenei – whose 37-year reign was brought to an end in February by the first airstrike of the war launched by the U.S. and Israel – in a show of public devotion to the Islamic Republic's theocratic state and revolutionary zeal.