"How can they put a matter that violates Sharia to a referendum?"
Al-Zawahiri also criticised Muslims for failing to support Muslim fighters in Iraq and other regions.
"I call upon the Muslim nation to fear Allah's question [at judgement day] about its failure to support its brothers of the Mujahidin and [urge it] not to withhold men and money, which is the mainstay of a war," he said.
Al-Zawahiri said that the fight against the Iraqi government and the US-led coalition forces is bearing fruit.
"I urge all Muslims to hurry to the battlefields of Jihad (holy war), especially in Iraq," he said
The comments are part of a two-part series where al-Zawahiri is to answer about 100 questions put to him via online forums.
"The situation in Iraq heralds an imminent victory of Islam and the defeat of the Crusaders and those who stand under their flag," he said.
Al-Zawahiri called on the various fighters operating in the country to unite behind the "more advanced" al-Qaeda-backed "Islamic State of Iraq".
In the first part of the message released last Friday, al-Zawahiri commemorated the fifth anniversary of the US-led invasion of Iraq with a call to Muslims to make Iraq a "fortress of Islam".