Foreign ministers of the Organisation of Islamic Cooperation, in a statement, said it "condemns the aggressions against the missions of the Kingdom of Saudi Arabia in Tehran and Mashhad".
The statement came in an extraordinary meeting requested by Saudi Arabia after protesters in Iran burned Riyadh's embassy in Tehran and a consulate in the second city of Mashhad.
Such "aggressions" contravened international law as well as the OIC charter, said the communique, which member state Iran rejected.
The violence against Riyadh's missions occurred after the kingdom executed dissident Shiite cleric Nimr al-Nimr, a driving force behind anti-government protests.
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Nimr was one of four Shiites put to death on January 2 alongside 43 Sunnis. All were convicted of "terrorism".
The 57-member OIC said it "rejects and condemns Iran's inflammatory statements" over the executions, "considering those statements a blatant interference in the internal affairs" of Saudi Arabia.
It also denounced "Iran's interference in the internal affairs of the states of the region and other member states (including Bahrain, Yemen and Syria and Somalia) and its continued support for terrorism".