Al Qaeda has named a new leader, Ayman al Zawahiri, an Egyptian who managed the group during a period of deadly innovation but is said to lack the charisma of his predecessor, Osama bin Laden.

As al Qaeda's new leader, Mr. Zawahiri faces a struggle to emulate bin Laden's influence over his followers and supporters, and will have to navigate a period of exceptional change for the international terrorist group and for the Muslim world, its main theater of operations.

Popular uprisings have brought down regimes in Tunisia and Egypt that suppressed their Islamist movements, and have led to open conflict in Libya, Yemen and Syria, where authorities have long confronted Muslim extremists—radically altering a political landscape once seen as fertile for Islamist extremism.

At the same time, the emergence of mass movements supporting democratic change has created an alternative to al Qaeda for young Arabs angry at the limited freedom and opportunities available in their countries.

Ayman al-Zawahiri, who was named leader of al Qaeda following bin Laden's death, in an undated video. Al-Jazeera/Associated Press

Meanwhile, the success of al Qaeda's affiliate in Yemen, al Qaeda in the Arabian Peninsula, in devising plots against Western targets, has revealed a gradual shift of momentum inside the group, from its older leadership in Afghanistan and Pakistan, including Mr. Zawahiri, to its younger commanders in Yemen and Somalia, said analysts.

"The general command of al Qaeda, after completing consultations, announces the assumption of Sheikh Ayman al Zawahiri of responsibility for commanding the group," said an announcement posted Thursday on the Ansar al Mujahidin website, which has links to al Qaeda.

The statement pledged that al Qaeda would continue its jihad against Western targets following the death of its leader, who held iconic status among Islamist extremists across the globe.

Mr. Zawahiri worked closely with bin Laden starting in the mid-1980s, when the pair backed the fight against Soviet forces in Afghanistan. They forged a partnership that spawned the Sept. 11, 2001, attacks on the U.S. The U.S. Federal Bureau of Investigation lists Mr. Zawahiri as one of its 10 most wanted terrorists, assigning a $25 million bounty—higher than that offered for any other fugitive.

Over the years, the Egyptian surgeon, who turns 60 on Sunday, developed a reputation as a focused operational leader and radical theologian.

However, a prickly character and a lack of personal charisma mean Mr. Zawahiri might struggle to gain the authority of bin Laden, who was shot dead by U.S. Navy SEALs in his Pakistani hideout in May, according to experts on Islamist terrorism.

"He does not project the calmness and command of bin Laden and he has in the past descended into vitriolic arguments with colleagues," said Magnus Ranstorp, research director at the Center for Asymmetric Threat Studies, at the Swedish National Defense College. "This will be a real challenge for him in maintaining that cohesion and maintaining control."

He might also struggle to overcome a perception that he is focused too closely on his home country of Egypt, at the expense of other countries, said Mr. Ranstorp.

Born into a prosperous Cairo family, Mr. Zawahiri formed a militant group while still at school, aiming to establish a radical Islamic state. As a founding member of the group Islamic Jihad, he pioneered the use of suicide bombing, and was imprisoned for three years in an Egyptian jail.

The group was badly bruised by its long fight against Egyptian police, and ultimately Mr. Zawahiri and his cohorts fled Egypt. Their experience and expertise eventually made them a valuable core of al Qaeda. In 1998, after Mr. Zawahiri joined bin Laden in Afghanistan, they announced a new jihadist front whose founding manifesto was to kill Americans wherever possible.

That mission was quickly turned into action, with attacks on U.S. embassies in Kenya and Tanzania that killed over 200 in 1998, and an attack in 2000 on a U.S. naval destroyer harboring in Aden, Yemen, that killed 17 sailors. The U.S. has accused Mr. Zawahiri of helping to plot all three operations.

In the run-up to the Sept. 11 attacks, Mr. Zawahiri was the chief planner and tactician for al Qaeda, while bin Laden was its public voice.

Mr. Zawahiri repeatedly berated underlings for security lapses, and fought off others who disapproved of plans to attack the U.S. instead of Arab regimes.

Former jihadists who worked with both men have said Mr. Zawahiri was far more security-conscious than bin Laden, perhaps due to his experience working underground in Egypt. He shunned the use of email and would only accept letters that were hand-carried.

In announcing Mr. Zawahiri's accession, al Qaeda confirmed several "principles," including "inciting the Muslim nation to...perform jihad against the infidel invaders who have aggressed the Muslim nations," singling out Israel and the U.S., according to the statement on the Ansar al Mujahidin website.

It also affirmed its support for the people of Afghanistan and Arab countries. "We assure our brothers in Afghanistan we are with them," said the statement. "We support the uprisings of our oppressed Muslim peoples that have risen in the face of dark, corrupt tyrants…in Egypt and Tunisia and Libya and Yemen and Damascus and Morocco."



—Nour Malas contributed to this article.