According to the source, the attack was carried out from the roof of the nearby American University.
Earlier in the day Monday, the commander of the Central Egyptian police, Majid Mustafa, was wounded in the shoulder by firing buckshot.
According to official data in three days of violence between police and demonstrators, 24 officers and 80 soldiers of Interior Ministry were injured.
The unrest in the Egyptian capital during the weekend are the most violent since the fall of Hosni Mubarak in February 2011. The demonstrators are demanding that the Supreme Council of the Armed Forces returns its powers to a civilian government.
The death toll more than 30 dead and 2,000 injured.