Paramedic Mohammed Sultan put the total number of wounded in the attack at 15, including nine members of the security forces and six protesters.
The witnesses said attackers also threw Molotov cocktails at protesters' tents, setting some on fire. Footage on Egyptian TV stations showed more people coming to join the protests.
The sit-in was started Dec. 4 by opponents of Islamist President Mohammed Morsi. They are demanding the annulment of Egypt's new Islamist-backed constitution. The document deeply polarized Egyptians but passed by a 64 percent "yes" vote in a referendum in which around 33 percent of voters participated. Critics called the process flawed.
The political conflict has been accompanied by street violence. At least 10 people died in clashes outside the palace on Dec. 5 that broke out when supporters of Morsi attacked the sit-in. Some were reportedly killed by gunfire.
On Dec. 31, gunmen shot and critically wounded a well-known activist at the site of another sit-in in downtown Cairo's Tahrir square. Police said they arrested a cafe owner who told them that he fired on the square after people manning makeshift checkpoints there searched his car and shot at him.
The current attack comes two weeks before the anniversary of the Jan. 25 start of the 2011 uprising that overthrew Mubarak. Activists opposed to Morsi are expected to organize large protests that day.
Earlier Saturday, a Cairo court ordered a religious TV program hosted by a fiery preacher off the air on charges of libeling and defaming a well-known actress, one of three legal reverses that day suffered by Islamists in cases related to the media.
The court ruled that the program "In The Scale" be suspended for 30 days following a lawsuit by Elham Chahine. A widely circulated video clip shows the program's host, Abdullah Badr, accusing Chahine of practicing "prostitution" and "teaching Egyptians how to strip naked, make love and commit adultery."
"Go ask God for forgiveness for your scandals," he says in the August interview. Chahine's lawyer said in court that the actress had been exposed to "insults, cursing and humiliation."
Last month, Badr was sentenced to a year in jail over the same charges. The program is aired on el-Hafiz TV, one of several networks associated with the ultraconservative Salafi Islamist movement.
In another case, a court dropped one of several lawsuits filed against popular satirist Bassem Youssef, known as Egypt's Jon Stewart. Youssef had been accused of "corrupting morals" and violating "religious principles" in his show, "The Program," in which he frequently mocks ultraconservative clerics and Islamists.
He still faces trial on March 9 on charges of insulting President Mohammed Morsi, a lawsuit that was leveled by lawyers associated with the Islamist group from which Morsi hails, the Muslim Brotherhood. This is one of many cases brought against media personalities who criticized the president. Morsi's office maintains that the president has nothing to do with the legal procedures against his critics.
In a separate court cast, a court ruled that Dream TV, a private liberal-leaning network that is sharply critical of the Brotherhood, could resume broadcasting. Egypt's Islamist minister of information Salah Abdel-Makksoud suspended it for an alleged zoning violation and broadcasting from outside an authorized area.
Neither Badr, Youssef nor Dream TV could immediately be reached for comment.