“The general leadership of Al Qaeda group, after the completion of consultation, announces that Sheikh Dr Ayman Zawahri, may God give him success, has assumed responsibility for command of the group,” Islamist website Ansar al-Mujahideen (Followers of the Holy Warriors) said in a posted statement.
Mr. Zawahiri was widely expected to succeed Osama Bin Laden who was killed in a firefight with US forces in Pakistan on May 2, 2011.
Mr. Zawahiri has been the brain behind Mr. 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.
“The man who terrified America in his life will continue to terrify it after his death,” he said in the video message titled “The Noble Knight Dismounted,” which was posted on jihadist online forums on June 8.
“You will continue to be troubled by his famous vow: You shall not dream of security until we enjoy it and until you depart the Muslims’ lands,” added the Egyptian militant, who was in white robes and a turban with a machinegun behind him.
The senior Al Qaeda leader vowed to make sacrifices needed to “deprive America of security.”
“We will pursue the jihad until we expel the invaders from Muslim lands,” he said.
Mr. Zawahiri also vowed allegiance to the leader of Taliban, Mullah Omar.
“We renew our allegiance to the leader of the believers, Mullah Mohammad Omar,” said Mr. Zawahiri. Mullar Omar, the one-eyed Taliban leader, has a $10-million US bounty on his head.
"We promise him obedience ... in jihad for Allah and to set up sharia (Islamic) law," he added.
Born into an upper-class family of scholars and doctors in an upscale Cairo neighborhood, the cerebral Egyptian, who is in his late-50s, is second after Mr. Bin Laden on the FBI “most wanted terrorists” list.
Both Mr. Bin laden and Mr. Zawahiri eluded capture when US-led forces toppled Afghanistan’s Taliban government in late 2001 after Al Qaeda’s September 11 attacks on US cities.
Bespectacled, with grey hair and a grey beard, Mr. Zawahiri won prominence in November 2008, when he attacked then US President-elect Obama as a “house Negro,” a racially-charged term used by 1960s black American Muslim leader Malcolm X to describe black slaves loyal to white masters.
In a subsequent video, in September 2009, Mr. Zawahiri returned to the attack on President Obama, saying he was no different from his predecessor George W. Bush.
“America has come with a new deceptive face ... It plants the same dagger as Bush and his predecessors did. Mr. Obama has resorted to the policies of his predecessors in lying and selling illusions,” said Mr. Zawahiri, clad in white robe and turban.
Like Mr. Bin Laden, Mr. Zawahiri has long been thought to be hiding along the rugged Afghan-Pakistan border.
Analysts have described Mr. Zawahiri as Al Qaeda’s chief organizer and Mr. Bin Laden’s closest mentor. “Mr. Ayman is for Bin Laden like the brain to the body,” Montasser al-Zayat, a lawyer in Cairo who once represented Mr. Zawahiri, told Reuters.
Mr. Zawahiri and Mr. 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. But the alliance was not Mr. Zawahiri’s first foray into militancy.
Born in 1951, he was the son of a pharmacology professor and grandson of the grand imam of Al-Azhar, one of the most important centers of Sunni Islam in the Muslim world.
He graduated from Egypt’s most prestigious medical school in 1974 and did a second degree in surgery. By then he was involved with the Muslim Brotherhood, a non-violent group seeking the creation of a single Islamic state.
When the militant Egyptian Islamist Jihad was founded in 1973, he joined. When members posed as soldiers and assassinated President Anwar el-Sadat in 1981, he was among 301 people arrested.
He went on trial but was cleared. He did, though, spend three years in jail for having an unlicensed pistol. On his release, Mr. Zawahiri made his way to Pakistan where he worked with the Red Crescent treating fighters wounded in the Afghan war.
Taking over the leadership of Jihad in Egypt in 1993, he was a key figure in a campaign in the mid-1990s to set up a purist Islamist state there; more than 1,200 Egyptians died in that campaign.
In 1999, an Egyptian military court sentenced Mr. Zawahiri to death in absentia. He has also been indicted in connection with the 1998 bombings of US embassies in Kenya and Tanzania.
Days after those bombings, he telephoned a Pakistani reporter, denying responsibility, but urging Muslims to “continue their jihad against the Americans and Jews.”
Mr. Zawahiri’s wife and three daughters were reported killed in a bombing strike on the Afghan city of Kandahar, the stronghold of the Taliban, in early December 2001.
Mr. Zawahiri has appeared regularly in a series of video or audio messages since then, criticizing the US war in Iraq, praising the Taliban and the suicide bombers who attack London in 2005 and urging Muslims to help victims of an earthquake in Pakistan.
Mr. Zawahiri has repeatedly called for Al Qaeda to seize control of a state, a goal the group has never come close to despite its alliance with Afghanistan's late 1990s Taliban rulers.
“Confronting the enemies of Islam and waging a jihad against them require a Muslim authority, established on Muslim territory that raises the banner of jihad and rallies Muslims around it,” he wrote in a 2001 essay, “Knights Under The Prophet’s Banner.”
“If we do not achieve this goal, our actions will be nothing more than small scale harassment,” he said.
(Mustapha Ajbaili, a senior editor at Al Arabiya English