CAIRO - Thousands of supporters of Egypt's Muslim Brotherhood stood their ground in Cairo on Sunday, saying they would not leave the streets despite "massacres" by security forces who shot dozens of them dead.Egypt's ambulance service said 72 people were killed in Saturday's violence at a Cairo vigil by supporters of deposed President Mohamed Morsi, triggering global anxiety that the Arab world's most populous country risked plunging into the abyss.Morsi's Brotherhood, which won repeated elections after the fall of autocrat Hosni Mubarak in 2011, has vowed not to leave the streets unless Morsi is restored to power. His supporters accuse the military of reversing the uprising that brought democracy to the most populous Arab state. "They will not be content until they bring back everything from the era of the corrupt, murderous security and intelligence state," senior Brotherhood official Essam el-Erian said on Facebook. "They've stepped up their efforts to do so by committing massacres never before seen in Egyptian history."Although Cairo was quiet on Sunday morning, violent clashes rattled the Suez Canal city of Port Said, with a 17-year-old youth killed in fighting between the pro- and anti-Morsi camps and a further 29 people injured, security sources said.The violence has deeply polarized Egypt, with its secular and liberal elite so far showing little sympathy for the Brotherhood or reservations about the return to power of a military which ruled for 60 years before the 2011 uprising.However, in one of the first signs of doubt from within the interim cabinet installed after the military takeover, Deputy Prime Minister for Economic Affairs Ziad Bahaa El-Din said the government must not copy the "oppressive and exclusionary policies" of its foes."Our position must remain fixed on the need to provide legal guarantees not only for the members of the Brotherhood, but for every Egyptian citizen. Excessive force is not permitted," El-Din wrote on Facebook. And in another sign of unease, the Tamarud youth protest movement, which mobilized millions of people against Morsi and has fully backed the army, expressed alarm at an announcement that the interior minister was reviving the feared secret political police shut down after Mubarak was toppled.The killings took place the morning after mass demonstrations called by military chief Abdel Fattah al-Sisi to show public support for a crackdown on "terrorism", which the Brotherhood sees as justifying an onslaught against it.In an apparent show of support for the police, a smiling Sisi turned up at a graduation ceremony on Sunday broadcast live on state television, receiving a standing ovation from the recruits, all decked out in starched white uniforms.Interior Minister Mohamed Ibrahim hailed him as "Egypt's devoted son".The military insists it does not want to retain power and aims to hand over to full civilian rule with a "road map" to elections in about six months.But the very public role of Sisi as face of the new order has led to speculation that the next president could again be a military officer, like all of Egypt's rulers between 1952 and Morsi's election last year.On Sunday morning army vehicles still surrounded entrances and exits to the square in northeast Cairo where thousands of Morsi supporters have camped out for a month. Authorities have said they want to clear the activists off the streets, but they were still camped there. Some used pictures of Morsi to shield their heads from the sun. Women and children were resting at tents set near the main gathering area."We are right, legitimacy is on our side and hopefully at the end God will lead us to triumph and we will not give up," said Mostafa Ali, 29 from Nile delta town of Mansoura.