Masked gunmen attacked opposition protesters camped out at Cairo’s Tahrir Square early on Tuesday, firing birdshots 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 President over a disputed draft constitution. The charter has vastly polarized the nation and triggered some of the worst violence since Mohamed Morsy 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.

The protesters belong to the liberal opposition, which claims the draft of the charter restricts freedoms and gives Islamists vast influence over the running of the country. The draft, hurriedly adopted late last month by Morsy’s allies, is going to a nation-wide referendum on Saturday.

The dispute prompted hundreds of thousands of the President’s opponents to take to the streets in massive rallies, the largest from primarily secular groups since the uprising that toppled Hosni Mubarak last year. Morsy’s supporters responded with huge demonstrations of their own, which led to clashes in the streets that left at least six people dead and hundreds wounded.

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