It was the first explicit claim of responsibility for the blasts by the group headed by Osama bin Ladin.
Al-Zawahiri threatened the West with "more catastrophes" in retaliation for the policies of US President George W Bush and British Prime Minister Tony Blair.
In his tape, al-Zawahiri did not say outright that his group carried out the London bombings, but he said they were a direct response to Britain's foreign policies and its rejection of the truce that al-Qaida offered Europe in April 2004.
"I talk to you today about the blessed London battle which came as a slap to the face of the tyrannical, Crusader British arrogance," al-Zawahiri said. "It's a sip from the glass that the Muslims have been drinking from."
He said: "This blessed battle has transferred - like its glorious predecessors in New York, Washington and Madrid - the fight to the enemies' land, after many centuries of the battle being on our (Muslim) land and after (Western) troops have occupied our land in Chechnya, Afghanistan, Iraq and Palestine."
In a clear bid to turn Britons against the government, al-Zawahiri said: "Blair not only disregards the millions of people in Iraq and Afghanistan, but he does not care about you as he sends you to the inferno in Iraq and exposes you to death in your land because of his Crusader war against Islam."
Led by arrogance
Al-Zawahiri continued: "We have alerted and warned you, people of Crusade allies, but it appears that you want us to make you taste death in all its horribleness.
"So, taste some of what we have been made to taste.
"Did not the Lion of Islam, the Mujahid Shaikh Osama bin Laden, offer you a truce so that you might depart from Islamic lands?
UK Foreign Secretary Jack Straw
was singled out for criticism
"But you were obstinate and were led by arrogance to more crimes and your Foreign Secretary Jack Straw said that these proposals "deserve to be met with our contempt".
Al-Zawahiri continued to address Britons: "Rejoice the outcome of your governments' arrogance.
"Blair brought upon you disasters in the centre of your capital and we will bring upon you more.
"He is still fooling his people and insisting in his obstinacy to treat them as if they are idiots incapable of understanding."