Nine people have died in Cairo in overnight clashes between supporters and opponents of ousted President Mohammed Morsi, officials say.

Most of the victims were killed at a sit-in held by pro-Morsi demonstrators near Cairo University.

Morsi's family earlier accused the military of abducting him. He has been held at an undisclosed location without charge since army chief General Abdul Fattah al-Sisi announced his removal on July 3.

Morsi's Muslim Brotherhood movement has refused to recognize the new military-backed administration and continues to hold almost daily street protests.

Demonstrators, many of them from the Muslim Brotherhood, have been staging large rallies across Egypt.

The violence has claimed at least 100 lives since Morsi was deposed.