Sunni Islam's leading centre of learning, Al-Azhar, said the "vile terrorist attack" contradicted Islam and called for "uniting efforts to defeat terrorism and rid the world of its evil."
Tunisia said that the attacker, who police said held joint French-Tunisian citizenship, had committed an act of "extreme cowardice" and expressed solidarity with France against the "scourge of terrorism."
At least 84 people were killed when the gunman rammed a truck through a crowd of thousands celebrating Bastille Day on the French Riviera last evening. Scores more were wounded, 50 of them children.
"People who commit such ugly crimes are corrupt of the earth, and follow in the footsteps of Satan... And are cursed in this life and in the hereafter."
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The six Gulf Arab states issued a joint statement saying that they "strongly" condemned the "terrorist" act in Nice.
"The Gulf Cooperation Council states stand in solidarity with the French republic following this cowardly criminal incident whose perpetrators have been stripped of all moral and human values," the bloc's secretary general, Abdullatif al-Zayani, said.
Regional heavyweight Saudi Arabia issued its own statement condemning the "heinous terrorist" act, adding that it stands in "solidarity" with France and will "cooperate with it in confronting terrorist acts in all their forms."
Both Saudi Arabia and the UAE are members of a US-led coalition which has carried out an air war against the Islamic State jihadist group in Syria alongside France.
Egyptian President Abdel Fattah al-Sisi condemned "in the strongest terms the vile terrorist attack," his office said.
Arab League chief Ahmed Abul Gheit denounced the "craven terrorist attack," his spokesman said.