Military helicopters dropped Egyptian flags on protesters in the main sit-in supporting ousted President Mohamed Morsy on Friday afternoon.
The sit-in, at Rabea al-Adaweya in eastern Cairo, has been the focal point for pro-Morsy opposition to the 3 July "military coup" since protesters first set up camp there almost a month ago.
Protesters chanted against the army the flags were dropped.
"We do not want flags, we want Morsy in the square," the crowds could be heard shouting.
"Down with military rule," others chanted.