"Honourable members of the Fighting Islamic Group in Libya announce that they are joining the al-Qaeda group to continue the march of their brothers," al-Zawahri said in the statement produced by as-Sahab, the al-Qaeda's media arm.
He urged mujahideen in North Africa to topple the leaders of Libya, Tunisia, Algeria and Morocco.
"O nation of jihad, support your sons so that we defeat our enemies and rid our homeland of their slaves," he said, naming the leaders of the four North African Arab nations.
Al-Zawahiri presented Abu al-Laith, who appeared to be the leader of the new Libyan wing.
"We announce we are joining al-Qaeda as loyal soldiers," Abu al-Laith said in a 28-minute recording posted on an Islamic website on Saturday.
The Fighting Islamic Group first announced its presence in 1995, vowing to overthrow Muammar Gaddafi, the Libyan leader.
Its members have clashed with government forces for years and several of its leaders have been arrested in Libya and abroad.
In 2001, the group was added to a UN list of individuals and institutions "belonging to, or associated with, al-Qaeda".
'Enemy of Islam'
Al-Zawahiri described Gaddafi as an enemy of Islam and criticised him for abandoning his "weapons and equipment to ... crusader masters".
The statements were in reference to the Libyan leader's decision to end years of international isolation in 2003, when it promised to give up nuclear, chemical and biological weapons and accepted responsibility for the Lockerbie bombing in 1988.
Al-Zawahiri also urged members of Fatah, the Palestinian movement, backed by Mahmoud Abbas, the Palestinian president, to abandon his secular line and join Islamic-led fighters, in an apparent reference to Hamas and the Islamic Jihad.
"I urge ... Fatah and al-Aqsa [Martyrs] Brigades to confront their leadership which turned their organisation into a branch of the CIA and the Mossad," al-Zawahiri said.