CAIRO, Egypt -- Hundreds of Islamists rallied in Egypt's capital and across the country Friday, calling for a boycott of an upcoming constitutional referendum as clashes with security forces killed three people, officials said.

Friday has traditionally been a day of protests for the supporters of Islamist President Mohammed Morsi since he was toppled in a July 3 popularly backed military coup. This Friday, Islamist supporters of Morsi and his Muslim Brotherhood organization added a call to boycott of the Jan. 14-15 referendum on changes to the constitution drafted under the ousted president.

Egypt's military-backed interim government is seeking a comfortable "yes" majority vote and a strong turnout that would enshrine their legitimacy ahead of planned presidential and parliamentary elections.

In clashes Friday in the Mediterranean city of Alexandria, a street vendor was shot and killed as hundreds of Morsi supporters marched, Maj. Gen. Nasser el-Abd said. It wasn't clear who shot the vendor.

Protesters set fire to tires and trash cans, threw gasoline bombs, used flares and threw rocks at the security forces, who fired tear gas and chased them in side streets, according to a police statement. Two people were injured by bird shot, while protesters set a car ablaze and partially burnt a police vehicle, police said.

In Suez, gunfire killed two people and injured seven as clashes raged between Islamists and security forces, a health official said. The official spoke on condition of anonymity as he wasn't authorized to speak to journalists.

Waleed Gelany, an eyewitness to the clashes, said police fired tear gas even before the protesters attempted to march from a local mosque to the centre of the city. Gelany said soldiers fired live ammunition at protesters.

In the eastern Cairo district of Nasr City, protesters hurled stones and empty bottles at security forces, a security official said. He spoke on condition of anonymity as he was not allowed to brief journalists. In the southern city of Minya, Maj. Gen. Osama Metwalli said protesters threw gasoline bombs at the police, who responded with tear gas.

Police arrested at least 70 Morsi supporters Friday and confiscated firearms and ammunition, officials said. Health Ministry official Khaled el-Khatib confirmed the three deaths and said the clashes wounded 12 people nationwide.

In recent weeks, protests have become increasingly violent and the government designated the Brotherhood a terrorist group. At least 17 civilians were killed last Friday around Egypt, including 10 in the capital alone.

The Brotherhood-led coalition has promised to protest during the vote, raising fears of possible unrest during the poll. On Friday, military spokesman Col. Ahmed Mohammed Ali said at least 160,000 soldiers will deploy alongside the police to secure polling stations nationwide.

Ali said on his official Facebook page that forces were trained on practical ways to "disperse riots and deal with protests and attempts disrupt the vote."

Soccer matches in the country's Premier League also have been postponed until after the vote, said Azmy Megahed, a spokesman for the Egyptian Football Association. Matches resumed only two weeks ago after a nearly two-year hiatus following a deadly stadium riot that killed 74 people.