Cairo — Nine people were hurt when gunmen fired at protesters camping in Cairo's Tahrir Square on Tuesday, according to witnesses and Egyptian media, as the opposition called for a major demonstration it hopes will force President Mohamed Mursi to postpone a referendum on a new constitution.

Supporters of the Islamist leader, who want the vote to go ahead as planned on Saturday, were also gathering in the capital, setting the stage for further street confrontations in a political crisis that has divided the Arab world's most populous nation.

Meanwhile, the secretary-general of the committee overseeing Egypt's constitutional referendum on 15 December has threatened to step down if more violence erupts before or during polling.

Zaghloul El-Balshi, who was appointed by President Morsi, said he had instructed judges supervising the referendum to close polling stations and go home if violence erupts during the vote.

Egyptians should refrain from protesting and express their view on the constitution via the ballot box, he added.