The nine minute, 59 seconds-long tape, entitled "An Elegy to the Martyred Commander Abu Laith al-Libi," was issued by al-Sahab, the organization's media branch.
The video had English subtitles and was followed by a letter addressed to al Qaeda sympathizers and Muslims in general. Its authenticity could not be immediately verified, but SITE and the IntelCenter, two U.S. groups that monitor militant messages, also reported it.
Al-Libi is believed to have been killed by a missile from a U.S. Predator drone that struck his safehouse in Pakistan in late January. He was viewed as a top al Qaeda strategist in neighboring Afghanistan.
Pakistani intelligence considered him the operational commander of al Qaeda in the border region and one of the militant group's most high-profile figures after its leader, Osama bin Laden, and his deputy al-Zawahiri.
In the tape, Al-Zawahiri described Al-Libi as a "knight" of al Qaeda's holy war. He was "a mountain of Jihad and a lion," al-Zawahiri said.
The al Qaeda deputy implicitly threatened the U.S. and its allies, saying that new leaders would replace al-Libi to continue the militant group's fight. "You Americans and you the agents of the Americans: (al-Libi and others) are the pioneers of the march and the good omen of a new dawn," he said.
"Every time a martyr falls, another martyr grabs the banner from him, and every time a chief goes down in blood, another chief completes the march after him," al Qaeda's deputy chief said.
Al-Zawahiri, an Egyptian doctor, also lashed out at his former allies of the Islamic Jihad in Egypt who have recently called for revising the group's radical ideology. He described the group's leaders, currently held in Egyptian jails, as "hypocrites and followers of despots" for softening their stance.
The video released Wednesday was posted on a web site that usually releases al Qaeda messages. The web site also displayed a banner advertising an upcoming "interview" of al-Zawahiri by al-Sahab, with a picture of him in a white traditional Arab robe and black turban, seated before a bookshelf. Such advertisements are usually posted up to 72 hours before the message is released.
Al-Zawahiri is believed to play a large role in directing al Qaeda's strategy on the ground and issues frequent videos and audiotapes, often laying out the network's doctrinal line. He issued 15 video and audio messages in 2007, and is seen by many counterterrorism experts to be al Qaeda's operational chief, rather than bin Laden.
The message released Wednesday was the first tape or video issued by al-Zawahiri this year. It followed a tape issued on Feb. 6 by Mustafa Abu al-Yazeed, the self-proclaimed leader of the terror network's Afghanistan branch.
In the 12-minute video, Abu al-Yazeed said the death of al-Libi and other al Qaeda commanders only strengthened the group's resolve against "infidels."