Police fired tear gas at university students in Cairo on Sunday as they demonstrated in support of ousted president Mohammed Morsi, two days ahead of a referendum on a new constitution. Clashes between police and the protesters erupted in front of Cairo University, al-Azhar University and Ain Shams University, security officials said, adding that 19 students, including four women, were arrested. Sunday's clashes came as Egypt's military-installed authorities prepare for a referendum on a new constitution, which is due to take place on Tuesday and Wednesday. The referendum has been billed as the first step in Egypt's democratic transition after the military overthrew Morsi in July after protests against his one-year rule. The vote is to be followed by two elections -- to elect a new president and a new parliament, but the authorities have yet to decide which election will be held first. The referendum is also seen as a test of popularity for army chief General Abdel Fattah al-Sisi, who led the ouster of Morsi. On Saturday he called on the public to turn out "in strength" at the referendum this week. Experts say the country's interim rulers are hoping for a massive approval of the new constitution that would then allow Sisi to contest the presidential election. Sisi said on Saturday that he would run for the presidency if there was a "popular demand" for him to nominate himself, according to the state newspaper al-Ahram. The military has said it will deploy 160,000 soldiers to guard some 30,000 polling stations during the referendum.