"The Saudi government supports terrorists and takfiri (radical Sunni) extremists, while executing and suppressing critics inside the country," Foreign Ministry spokesman Hossein Jaber Ansari was quoted as saying by state news agency IRNA.
Lebanese council condemns execution
Lebanon's Supreme Islamic Shia Council condemned the execution of Nimr al-Nimr in Saudi Arabia on Saturday, saying it was a "grave mistake".
"The execution of Sheikh Nimr was an execution of reason, moderation and dialogue," the council's Vice President Sheikh Abdel Amir Qabalan said in a statement.
Execution intends to fuel Sunni-Shia strife: Iraq lawmaker
Execution of Nimr al-Nimr was intended to fuel Sunni-Shia strife and "set the region on fire", a lawmaker from Iraq's ruling Shia coalition told al-Sumaria TV on Saturday.
"This measure taken by the ruling family (of Saudi Arabia) aims at re-igniting the region, provoking sectarian fighting between Sunnis and Shias," Mohammed al-Sayhud said, according to the TV station's website.
Brother calls for calm
The execution Nimr al-Nimr could arouse anger among the minority's youths, his brother warned, while calling for calm.
"This action will spark anger of (Shia) youths" in Saudi Arabia, said Mohammed al-Nimr, calling instead for a "peaceful protest movement".
"We reject violence and clashing with authorities, just like the martyr sheikh rejected it."