EHRAN, Apr 21 - Bahrain's top Shiite cleric, Sheikh Issa Ghassem in a ceremony celebrating mid-Sha'ban and the birth of the savior of mankind, the twelfth Imam of Shia Muslims, Imam Mahdi (may God hasten his reappearance) in the holy mosque of Jamkaran said standing up to tyranny is the key to realization of Imam Mahdi's global reign.