"The spilling of their pure blood - including of the late cleric Sheikh Nimr al-Nimr, may his soul rest in peace - is an injustice and an aggression," Sistani said. Other leading Shiite clerics in Iraq have reacted with outrage to the execution on Saturday by the Saudi authorities of Nimr and other Shiite activists.

They were among a total of 47 people, most of them described by the interior ministry as involved in killings by Al-Qaeda. Moqtada al-Sadr, a well-known cleric who heads the Saraya al-Salam militia, said Nimr's execution was a "horrible attack" against Shiites and called for international condemnation.

Mohammed Taqi al-Mudaresi, another cleric who is based in the holy Shiite city of Karbala, took a harder line. "The execution of the martyr (Nimr) isn't just a declaration of war against the People of the House (Shiites) but against all Muslims," he said in a statement.

The executions also sparked a wave of anger in neighbouring Iran, where protesters firebombed the Saudi embassy in Tehran. The kingdom's mission in Iraq reopened on December 15, a quarter of a century after diplomatic ties were severed over the invasion of Kuwait.

The embassy is located in the fortified part of central Baghdad known as the "Green Zone", which is home ot most ley institutions and embassies. Ambassador Thamer al-Sabhan, who arrived in the country four days ago, posted a message on social media in which he said they are being "looked after by the Iraqi government".

Several clerics and protesters in Iraq have said that the embassy should be closed down and the envoy expelled over Nimr's execution.

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