Al-Zawahri, a surgeon by training, has long brought ideological fire as well as tactical and organizational cunning to al-Qaida, which has found itself increasingly decentralized and prone to internal disputes following its expulsion from Afghanistan after its invasion by U.S. forces in 2001.

Thursday’s announcement comes more than six weeks after the U.S. killed bin Laden in a May 2 raid on his home in Pakistan. Al-Zawahri pledged to avenge the death of the al-Qaida founder and mastermind of the Sept. 11 terror attacks and to continue the terror network’s campaign of attacks against the U.S. and other Western interests.

"The general command of al-Qaida, after completing consultations, decided that the sheik doctor Abu Mohammed Ayman al-Zawahri take the responsibility and be in charge of the group," said a statement purportedly by al-Qaida and posted on militant websites, including several known to be affiliated with the group.

Al-Qaida gave no details about the selection process for bin Laden’s successor but said that it was the best tribute to the memory of its "martyrs."

Al-Zawahri, who turns 60 on Sunday and has a $25 million bounty on his head, has been behind the use of suicide bombings and the independent militant cells that have become the network’s trademarks. But U.S. intelligence officials have said that some al-Qaida members find al-Zawahri to be a controlling micromanager who lacks bin Laden’s populist appeal.

He also faces significant challenges in promoting al-Qaida’s agenda of a religiously led state spanning the Muslim world after finding itself sidelined in the wake of popular revolts that have been driven by aspirations for Western-style democracy instead.

Al-Zawahri has been in hiding for nearly 10 years and is widely believed to be near the Pakistan-Afghanistan border. He has appeared in dozens of videos and audiotapes in recent years, increasingly becoming the face of al-Qaida as bin Laden kept a lower profile.

Most of his pronouncements on the videos and audiotapes show him to be a man consumed by deep hatred for the West, particularly the United States, and Israel.

Al-Zawahri had been considered the most likely successor because of his longtime collaboration with bin Laden. Analysts had said that few were likely to challenge the al-Qaida deputy leader for the top spot despite some reservations.

Many predicted he would step up attacks to prove himself.

"He was a given leader from the outset. But he doesn’t have the same iconic status or personality as bin Laden," said Magnus Ranstorp, a terror analyst at the Royal Swedish Defense College. "He will focus on attacking the West in a big way. To avenge (bin Laden’s death), but also to make himself … even more effective and relevant."

Al-Zawahri and bin Laden first crossed paths in the late 1980s in the caves of Afghanistan, where the Egyptian reportedly provided medical treatment to bin Laden and other Islamic fighters battling Soviet forces. Their alliance would develop years later into the terror network blamed for America’s worst terror attack in its history.

In a videotaped eulogy released earlier this month, al-Zawahri warned that America still faces an international community of Muslims that seek to destroy it.

"Today, praise God, America is not facing an individual, a group or a faction," he said, wearing a white robe and turban with an assault rifle leaned on a wall behind him. "It is facing a nation that is in revolt, having risen from its lethargy to a renaissance of jihad."

Al-Zawahri also heaped praise on bin Laden and criticized the U.S. for burying him at sea.

"He went to his God as a martyr, the man who terrified America while alive and terrifies it in death, so much so that they trembled at the idea of his having a tomb," he said.

Al-Zawahri is the son of an upper middle class Egyptian family of doctors and scholars. His father was a pharmacology professor at Cairo University’s medical school and his grandfather was the grand imam of Al-Azhar University, Sunni Islam’s formal seat of learning.

At the age of 15, he founded his first underground cell of high school students to oppose the Egyptian government. He continued his militant activities while earning his medical degree, later merging his cell with other militants to form Islamic Jihad.

Al-Zawahri served three years in an Egyptian prison before heading to Afghanistan in 1984 to fight the Soviets, where he linked up with bin Laden. Al-Zawahri later followed bin Laden to Sudan and then back to Afghanistan, where they found a safe haven under the radical Taliban regime.

Soon after came the 1998 bombings of the U.S. embassies in Africa, followed by the 2000 suicide bombing of the USS Cole off the coast of Yemen, an attack al-Zawahri is believed to have helped organize.

In a 2001 treatise, he set down the long-term strategy for the jihadi movement — to inflict "as many casualties as possible" on the Americans.

"Pursuing the Americans and Jews is not an impossible task," he wrote. "Killing them is not impossible, whether by a bullet, a knife stab, a bomb or a strike with an iron bar."

Al-Zawahri’s hatred for Americans has also become deeply personal: His wife and at least two of their six children were killed in a U.S. airstrike following the 2001 U.S. invasion of Afghanistan after the 9-11 attacks.

Al-Zawahri has worked in the years since to rebuild the organization’s leadership in the Afghan-Pakistan border. Al-Qaida has inspired or had a direct hand in attacks in North Africa, Saudi Arabia, Yemen, Pakistan, the 2004 train bombings in Madrid and the 2005 transit bombings in London.

The CIA came close to capturing him in 2003 and killing him in 2004 — both times in Pakistan. In December 2009, they thought they were again close only to be tricked by a double agent who blew himself up, killing seven agency employees and wounding six more in Khost, Afghanistan.

The statement announcing his succession was filled with the terror network’s usual rhetoric, vowing to continue the fight against what it called "conquering infidels, led by America and its stooge Israel, who attack the homes of Islam."

The al-Qaida statement also stated the group’s support for this year’s popular uprisings in the Middle East and North Africa. It vowed never to recognize the legitimacy of Israel and to support what it called the struggle of the Afghan people under the leadership of Taliban commander Mullah Omar against American occupation.

"We support the uprisings of oppressed Muslim nations in the face of corrupt and oppressive tyrants in Tunisia, Egypt, Yemen, Syria and Morocco and we encourage them along with the rest of the Muslim people to continue the struggle until all the corrupt oppressive regimes imposed by the west are removed," it said.

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Associated Press writer Karl Ritter in Stockholm, Sweden, contributed to this report.