Iranian protesters ransacked and set fire to the Saudi Embassy in Tehran on Saturday after Saudi Arabia executed an outspoken Shiite cleric who had criticized the kingdom’s treatment of its Shiite minority. The cleric, Sheikh Nimr al-Nimr, was among 47 men executed in Saudi Arabia on terrorism-related charges, drawing condemnation from Iran and its allies in the region, and sparking fears that sectarian tensions could rise across the Middle East. The executions coincided with increased attacks in Saudi Arabia by the jihadists of the Islamic State and an escalating rivalry between the Sunni monarchy and Shiite Iran that is playing out in conflicts in Syria, Yemen and elsewhere.