(CNN) Protests in Egypt over the proposed sale of two Red Sea islands to Saudi Arabia turned violent on Monday, with riot police using tear gas and rubber bullets against demonstrators across Cairo. Protesters angry over the island sale, Egypt's economy and police brutality chanted for the ouster of President Abdel-Fattah el-Sisi. Security forces detained an unknown number of protesters and also several journalists, who were later released. But police allowed dozens of Sisi supporters to gather in restricted squares and areas. Here's what's going on: Why are people unhappy? Many of the protesters do not want to see the two Red Sea islands, Tiran and Sanafir, turned over to the Saudis. Egyptians have also been troubled by other aspects of Sisi's presidency including the struggling economy and the detention of political dissidents.