On the tape, al-Zawahiri calls the leader of the Roman Catholic Church an "impostor" whose religion "is made of myths, like the Trinity, crucifixion, sacrifice and the original sin."
"This impostor, Benedict, reminds us of another impostor, his predecessor [Pope] Urban II from the 11th century in Clermont, France, who urged the people of Europe to kill the Muslims and launched the crusades," al-Zawahiri says.
Benedict, he says, "went beyond his way to defame us, [but] we will respond to his insult with kindness."
The Vatican said it had no comment on the tape. Benedict is at his summer residence in Castelgandolfo and is expected to make a public appearance Sunday. He is scheduled to return to the Vatican on October 3.
Al-Zawahiri also criticizes the United Nations for proposing to send peacekeepers into Darfur to quell the violence there. He calls the government in Khartoum, which has rejected the proposal but expressed interest in bolstering the African Union forces there, as "deceitful and hypocritical."
"Rise, O Muslim nation, to defend your land and your honors from the crusaders' aggression who are now hiding behind the masks of the United Nations," al-Zawahiri says. "Nothing will protect you but a jihadi popular war that will be led by the mujahedeen."
Al-Zawahiri closes his message by cautioning Muslims to reject governments that do not accept sharia, or Islamic law, and others that cooperate with the United States, particularly in Afghanistan and Iraq.
Lovely fellow. No wonder he's known as "Number Two". Fitting, isn't it?