Liu also noted that Chinese regulations require that all protesters apply and receive permission in advance, without clarifying whether that included the special zones.
Human rights groups have been critical of the plan
"Designating unilaterally 'protest zones' for demonstrators does not equate to respecting the right to demonstrate because in this situation control comes first and the right second," Nicholas Bequelin, a researcher with New York-based Human Rights Watch's Asia Division, said.
But Ni Jianping, director of the Shanghai Institute of American Studies, who lobbied Chinese leaders to set up the protest zones, said: "This will allow people to protest without disrupting the Olympics."
China has increased security for the Games, saying that it fears potential terrorist attacks by international groups or Muslim separtists from the west of the country.
"The common enemies of global society - that is to say terrorists - will want to come along with such a large number of people at an event like this, to try and promote their goals," Liu said.
About 110,000 police and soldiers, trained to deal with 39 different types of attack, have been deployed for the Games and surface-to-air missiles stand ready to target any potential threats.
Chinese authorities will also restrict aircraft landing and taking off during the Games opening ceremony due to security concerns.
The emphasis on security has seen huge queues of angry commuters trying to pass through security checks at subway stations, while foreign visitors have found it difficult to enter the country because of new visa requirements.
On Wednesday, authorities set up explosives detectors at airports across Xinjiang where officials say they have broken up several "terrorism groups".
However, critics say that China is using the Games as a pretext to crackdown on Muslim Turkic-speaking Uighurs in the region near the border with Pakistan and Afghanistan.