US web sites also released an audio message from Osama bin Laden, al-Qaeda's leader, on Thursday, which called for revenge against Pervez Musharraf, Pakistan's president.
Call for revenge
In his video message, Al-Zawahri appealed to al-Qaeda supporters to continue their fight against the West.
"Go forth ... to the mujahideen, bear them arms, back them, defend them and don't be intimidated by the power of America
for these two blessed attacks have revealed that it is a power of iron and fire, with no faith or morals or principle."
Al-Zawahri, who praised the actions of al-Qaeda-linked groups fighting in Afghanistan, north Africa, Somalia, Chechnya and Iraq, called on Pakistanis to avenge the killing of a rebel cleric sympathetic to Afghanistan's Taliban in July.
"Let the Pakistani army know that the killing of Abdul Rashid Ghazi and his students and the demolition of his mosque and two madrassas have soaked the history of the Pakistani army in shame ... which can only be washed away by retaliation against the killers of Abdul Rashid Ghazi and his students."
At least 75 of Ghazi's followers were killed in an assault on the Lal Masjid, a mosque and school complex in Islamabad.
Al-Zawahri gives what appears to be a new commentary, explaining how the United States was being defeated by Muslims around the world.
"What they claim is the strongest power in the history of mankind is today being defeated in front of the Muslim vanguard of jihad six years after New York and Washington," said al-Zawahri, who was speaking in front of a packed book case. An assault rifle leant against it.
"The coalition of the Crusaders has begun to fight the fight of the desperate ... by increasing its bombing of civilians to discourage them from supporting the Taliban..."
Bin Laden message
In separate audio tape comments released by US web site monitors on Thursday, Osama bin Laden, al-Qaeda's leader, said that Musharraf would face revenge for Ghazi's killing.
"We in al-Qaeda organisation call on God to witness that we will retaliate for the blood of ... Abdul Rashid Ghazi and those with him against Musharraf and those who help him, and for all the pure and innocent blood," bin Laden said in an excerpt aired on lauramansfield.com.
"So Pervez, his ministers, his soldiers and those who help him are all accomplices in spilling the blood of those of the Muslims who have been killed. He who helps him knowingly and willingly is an infidel like him," bin Laden was quoted as saying on the US Site Institute web monitor.