Protesters stormed and torched the Saudi Arabian Embassy in Tehran on Saturday after the execution of a prominent Shiite cleric ignited sectarian tensions across the already inflamed region, jeopardizing U.S. diplomacy aimed at tamping down conflicts in the Middle East.The unrest erupted after the announcement by Saudi authorities that Sheik Nimr Baqr al-Nimr, 56, was among a group of 47 people put to death in 12 Saudi cities.Some were killed by firing squad, others by beheading, according to a statement from Saudi Arabia’s Interior Ministry. Most were Sunnis accused of participating in al-Qaeda attacks in the kingdom.Nimr, however, was one of four Shiites put to death for political activism and the leading figure in the anti-government demonstrations that swept the mostly Shiite east of the country in 2011, inspired by the Arab Spring protests taking place elsewhere in the region.The death sentence was carried out despite widespread international appeals for clemency and repeated warnings from the kingdom’s archenemy Iran that there would be consequences if the popular cleric were killed.The U.S. State Department, which had refrained from publicly joining the appeals for Nimr’s life, said it had raised concerns at the highest levels of the Saudi government about the judicial process in Saudi Arabia. In a statement, it called on Saudi Arabia “to respect and protect human rights” and also to permit “peaceful expression of dissent.”“We are particularly concerned that the execution of prominent Shia cleric and political activist Nimr al-Nimr risks exacerbating sectarian tensions at a time when they urgently need to be reduced,” the State Department said in a statement. “In this context, we reiterate the need for leaders throughout the region to redouble efforts aimed at de-escalating regional tensions.”Shiites around the world expressed outrage, potentially complicating a sudden surge of U.S diplomacy aimed at bringing peace to the troubled region, according to Toby Matthiesen, an expert on Saudi Arabia at the University of Oxford.“Nimr had become a household name amongst Shiite Muslims around the world. Many had thought his execution would be a red line and would further inflame sectarian tensions,” he said. “So this will complicate a whole range of issues, from the Syrian crisis to Yemen.”Saudi Arabia and Iran are backing rival sides in Syria’s war, and their enmity risks derailing a diplomatic effort led by the United States and Russia to convene peace talks between the factions in Geneva this month.The two feuding powers also support opposing sides in the war in Yemen and more broadly find themselves in opposition in the deeply divided politics of the mixed Sunni-Shiite nations of Iraq and Lebanon.The Obama administration’s hopes that the conclusion last summer of an agreement limiting Iran’s nuclear program would help bridge the sectarian divide between Tehran and the United States’ biggest Arab ally were further diminished by the eruption of fury that followed Nimr’s death.