At least 13 people were reported injured on Friday as rival protests between Muslim Brotherhood supporters and the opposition umbrella group, the National Salvation Front, took place in Cairo.

The NSF took to Tahrir Square to rally their followers to vote "no" on the constitutional referendum, the day before in-country voting is scheduled to start. The pro-Muslim Brotherhood rally drew an estimated 2,000 people. Another 19 were reported injured in protests in Alexandria, Al-Arabiya reported, after stones were thrown between rival groups of protestors.

Opposition parties fear that the constitution is unfair to women, and is highly restrictive in the realm of religious freedom; the constitution only allows for the practice of Islam, Christianity, and Judaism. They also allege that the constitution was hurriedly push through the Islamist- and Muslim Brotherhood-dominated Egyptian Assembly, with little consideration for minority voices.

Voting begins for half of Egypt, including the major cities of Cairo and Alexandria, on Saturday December 15. The other half of the country will vote next week, on the 22nd.

Voting for the 586,000 Egyptians living outside Egypt began on Wednesday and continues until Saturday, and allegations of vote- tampering and fraud quickly began to come in.

On Friday Egyptian Foreign Ministry Spokesman Amir Roshdy vigorously denied rumors that 200,000 ballots had been pre-printed in Kuwait to rig the vote there, the daily Al-Masry al-Youm reported. Roshdy also denied that people who were seen handing out flyers printed on the official letterhead of the Egyptian Embassy in Kuwait encouraging people to vote 'yes' were in any way actually connected with the Embassy, claiming they had merely used the letterhead.

Anti-Islamist and opposition leaders used the last day before the referendum to try and get their message out to vote 'no.'

On Thursday, former presidential candidate and opposition leader Mohammed El-Baradei issued a statement saying “the constitution, drafted by a majority of Islamists, aims to turn Egypt into a religious state and represents a threat to basic freedoms and rights," the state run news agency Al-Ahram reported.

Another former presidential candidate, Abdel Moneim Aboul Fotouh, told his supporters to vote 'no,' citing concerns about social justice and the dominance of the military, in an interview with Al-Arabiya.

“We all demanded, from day one, that Egypt’s political system be presidential and parliamentary, only to find that the authorities of the president are almost unchanged,” Fotouh said.

The April 6 Youth Movement, also part of the opposition, posted online ads saying, “It’s you who will pay the price if you vote 'yes.' No to the constitution," Al-Arabiya translated.

The Muslim Brotherhood-allied Jama'a al-Islamiya and Construction and Development Parties announced they would be mobilizing to get out the 'yes' vote, said Egypt Independent.

U.S. State Department spokeswoman Victoria Nuland told the press on Thursday that the U.S. has "concerns" about the possible outbreak of more violence during the voting, but said that this was an opportunity for Egyptians to "exercise their democratic rights, and to do so peacefully and to participate in the future of their country."

"It’s also the responsibility of the government to provide a safe, transparent, and fair environment for voting, assuming that it goes forward," Nuland said. "So we call on Egyptians [and] Egypt’s political leaders of all stripes to make clear to their supporters that violence of any kind during this polling is unacceptable. And we call on the Egyptian people to take all possible measures to avoid confrontation and violence."