Egyptian-born doctor and surgeon Ayman al-Zawahri is al Qaeda's second-in-command expected to succeed Osama bin Laden following his killing in a firefight with U.S. forces in Pakistan, Reuters reports.

Zawahri has been the brains behind bin Laden and his al Qaeda network, and at times its most public face, repeatedly denouncing the United States and its allies in video messages. In the latest monitored by the SITE Intelligence Group last month, he urged Muslims to fight NATO and American forces in Libya.

"I want to direct the attention of our Muslim brothers in Libya, Tunisia, Algeria, and the rest of the Muslim countries, that if the Americans and the NATO forces enter Libya then their neighbors in Egypt and Tunisia and Algeria and the rest of the Muslim countries should rise up and fight both the mercenaries of Gaddafi and the rest of NATO," Zawahri said.

Born into an upper-class family of scholars and doctors in an upscale Cairo neighborhood, the cerebral Egyptian in his late-50s is second after bin Laden on the FBI "most wanted terrorists" list.

Analysts have described Zawahri as al Qaeda's chief organizer and bin Laden's closest mentor. "Ayman is for bin Laden like the brain to the body," said Montasser al-Zayat, a lawyer in Cairo who once represented Zawahri.

Zawahri and bin Laden met in the mid-1980s when both were in the Pakistani city of Peshawar to support guerrillas fighting the Soviets in Afghanistan, and worked closely thereafter.