Zawahiri, who will turn 60 this Sunday, had been a long-standing deputy to bin Laden, a Saudi whom American commandos shot dead at his seemingly secure hideout in Pakistan May 2.
Considered the brain behind Al Qaeda, Zawahiri carries a reward of $25 million announced by the US government. US officials say he has for years held operational responsibilities in the Al Qaeda because of the global hunt for bin Laden.
Al Qaeda's general command said in a statement posted on jehadi websites Thursday: "Sheikh Ayman al-Zawahiri...assumed responsibility as the group's amir (leader)".
The statement warned that the Al Qaeda would continue to fight "the holy war against the US and Israel" under Zawahiri's direction.
The group would fight "until all invading armies leave the land of Islam", it said.
Separately, BBC quoted Zawahiri as saying: "Today, and thanks be to god, America is not facing an individual or a group but a rebelling nation, which has awoken from its sleep in a jehadist renaissance."
Only a week ago, the Egyptian had appeared in a video message posted on the internet to say Osama had become a "martyr" and "we must continue on his path of jehad".
He warned that bin Laden would continue to "terrify" the US from beyond the grave.
In the June 8 video, Zawahiri called Pakistan, where bin Laden was killed amid speculation over Islamabad's role in the killing, a "colony" of the US -- "bought and paid for".
"Its government and army must be fought against," he said.
Nevertheless, many believe that Zawahiri is holed up in Pakistan, perhaps close to the border with Afghanistan, probably with help from a section of the Pakistani intelligence.
Zawahiri was wanted in the US even before the September 2001 terror attack targeting New York's World Trade Centre and the Pentagon.
He was indicted in absentia in 1999 for the 1998 bombings of US embassies in Tanzania and Kenya that killed 224 people. He was also considered the mastermind of the 2000 bombing of the USS Cole in Yemen which killed 17 sailors.
He has been blamed for the 1993 attack on the World Trade Centre in New York and the 1997 massacre of 58 tourists in Egypt.
Zawahiri also took responsibility for the devastating 2005 bombing on London's transport system.
Zawahiri was one of hundreds of Islamic militants put on trial in Egypt after the 1981 assassination of president Anwar Sadat. He was freed three years later.
He left for Saudi Arabia and eventually settled down in Pakistan, from where he joined the anti-Soviet mujahideen war. Once the Soviets went home he turned against the US -- with terrible consequences.
