More than one hundred people attacked protesters in Tahrir Square at dawn on Thursday, leaving two people injured.
The unknown assailants stormed the square from the direction of Talaat Harb Street. They removed barriers blocking traffic, set fire to tents and attacked protesters, state news agency MENA said.
Eyewitnesses said more than 100 people, some armed and riding motorcycles, others in pickup trucks, entered the square and attacked the protesters.
Two protesters were injured and received treatment in the square's field hospital.
On Wednesday, 53 protesters were arrested when security forces entered the square to remove barriers which were blocking the flow of traffic, MENA reported.
Protesters later replaced the barriers at the entrances to the square.
A number of protesters have been holding a sit-in in the square since 22 November when President Morsi issued a constitutional declaration that critics say rendered his decisions above judicial intervention.
Protesters prevented traffic from entering the square by blocking the entrances with barbed wire and metal barriers.
On 1 March, a police vehicle was reportedly set on fire by protesters near Abdel-Moneim Riyad Square, one of the entrances to Tahrir Square, after security forces removed tents from the square. Protesters reblocked the entrances to the square a couple of days later.