CAIRO — Masked gunmen attacked opposition protesters camped out at Cairo's Tahrir Square early on Tuesday, firing birdshot at them and wounding nine people, security officials said.

The attack stoked tensions just hours ahead of rival mass rallies in the Egyptian capital by supporters and opponents of the country's Islamist president over a disputed draft constitution. The charter has vastly polarized the nation and triggered some of the worst violence since Mohammed Morsi took office in June as Egypt's first freely elected president.

It was unclear who was behind the pre-dawn attack on the protesters who have been staging a sit-in at Tahrir for nearly three weeks, said the officials, speaking on condition of anonymity because they were not authorized to talk to the media.