Masked gunmen attacked protesters camped outside the presidential palace, hurling firebombs at their tents and firing birdshot in clashes that left policemen and civilians injured, a security official said on Sunday.
The gunmen threw petrol bombs overnight and set fire to the tents of protesters—opponents of Islamist President Mohamed Morsi—who have been camping out for more than a month.
When police intervened the gunmen fired birdshot into the crowd, the official said.
The health ministry said seven policemen, including a senior officer, and 16 civilians were injured in the clashes.
A security official said two gunmen were later arrested.
The protesters have been demanding that Morsi annul an Islamist-backed constitution that has polarized the country.
Last month, the document was approved by 64 percent of voters in a referendum, but only 33 percent of the eligible electorate turned out.
In November, Morsi issued a decree allowing for the constitution to be rushed through which led to deadly clashes sparking the worst political crisis since he took office in June.
The opposition fears the Islamists are riding roughshod over civil, political and human rights and the rights of women.