Unidentified assailants stormed Tahrir Square at dawn on Thursday, attacking protesters and opening the square for traffic.
Eyewitnesses said that more than 100 people, some of whom were armed, riding motorcycles and pickup trucks stormed the square from the direction of Talaat Harb Street.
They set fire to tents, while chasing and beating protesters.
Two protesters were injured, said eye witnesses, and treated at a field hospital near Tahrir Square.
Security forces had briefly opened Tahrir Square for traffic on Wednesday, removing barbed wire and iron barriers blocking the road.
But the square was closed again later that day when protesters replaced metal barriers and razor wire at the entrances, all the while chanting against the Interior Ministry and President Mohamed Morsy.
Traffic was jammed in surrounding roadways, with Bostan and Qasr al-Nil Streets once again seeing two-way traffic, as has been the case since the closure of Tahrir Square to traffic since November.
On Wednesday, security forces arrested 53 people on charges of arms possession after protesters allegedly hooked sacks filled with iron to electrical wires near the Arab League of Nations building to shock security forces if they attempted to open the square.
Street cleaners also removed the burned husk of a police vehicle torched in the square.