(Reuters) -

Thousands of supporters of Egypt's Muslim Brotherhood stood their ground outside a Cairo mosque on Sunday a day after at least 72 were gunned down by security forces, braced for a move against them by the army chief behind the overthrow of Islamist president Mohamed Mursi.

General Abdel Fattah al-Sisi made his first appearance since Saturday's bloodshed, smiling before television cameras at a police graduation ceremony, recruits decked out in starched white uniforms.

He received a standing ovation and was hailed by Interior Minister Mohamed Ibrahim as "Egypt's devoted son". Fawning coverage in state and private media reflected Sisi's rising political star, in a country ruled by former military officers for six decades before Mursi's election in 2012.

Saturday's dawn killings, following a day of rival mass rallies, triggered global anxiety that the Arab world's most populous and influential country risked broader conflagration.

The Brotherhood accuses the military of turning back the clock on the 2011 uprising that toppled Hosni Mubarak, and demands that Mursi, Egypt's first freely elected president, be reinstated.

Mursi has been in military detention since his July 3 overthrow and the military-backed interim government has placed him under investigation on charges including murder. Authorities also say they will move soon to clear the Brotherhood's tent vigil.

"It's a source of terrorism that's threatening the whole society, and that's being confirmed by the day," Mostafa Hegazy, adviser to interim President Adli Mansour, told reporters.

Army vehicles surrounded entrances to the square outside the Rabaa al-Adawiya mosque in northern Cairo on Sunday, where Brotherhood supporters used pictures of the bearded Mursi to shelter from the fierce sun.

"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 the Nile delta town of Mansoura.

"MASSACRES"

The Interior Ministry has rejected eyewitness accounts that police opened fire on the crowds and a public prosecutor has launched a probe into the violence, investigating 72 suspects for an array of crimes including murder and blocking streets.

Cairo was quiet on Sunday, but violent clashes rattled the Suez Canal city of Port Said, where a 17-year-old youth was killed in fighting between pro- and anti-Mursi camps and a further 29 people were injured, security sources said.

The violence has polarized Egypt, with its secular and liberal elite showing little sympathy for the Brotherhood or reservations about the military's return to the political frontline.

Speaking to Reuters, interim Foreign Minister Nabil Fahmy said deepening divisions would lead to "more tragedies". He blamed the Brotherhood for the violence, but said they should be part of the country's political future.

"If they decide to withdraw from politics, it will be disappointing, if they decide to pursue violence, then you are looking at a completely different confrontation," Fahmy said. "Even if I personally reject their positions or ideology, they have to find their place in Egypt's political life."

Egyptian officials said EU foreign policy chief Catherine Ashton was due to arrive in Cairo and would meet a number of political leaders, including Fahmy, on Monday.

In a first sign 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" policies of its foes. "Excessive force is not permitted," Bahaa El-Din wrote on Facebook.

The Tamarud youth protest movement, which mobilized millions of people against Mursi and has fully backed the army, also expressed alarm at an announcement by Interior Minister Ibrahim that he was reviving Mubarak's hated secret political police, shut down after he was toppled.

"CAN'T REWRITE HISTORY"

The military says it does not want to retain power and aims to hand over to full civilian rule with a "road map" to parliamentary elections in about six months. But the very public role of Sisi as face of the new order has sowed doubt.

The United States, which gives more than $1 billion a year in military aid to Egypt, urged its Middle East ally to pull "back from the brink" and respect the right to peaceful protest.

New York-based Human Rights Watch said the Saturday killings suggested a "shocking willingness" by police and politicians to ratchet up violence against backers of Mursi. U.N. human rights chief Navi Pillay said confrontation was "leading to disaster".

"Egypt stands at a crossroads," Pillay said in a statement. "The future of this great country that gave so much to civilization depends on how its citizens and authorities act over the following days and months."

Close to 300 people have died in violence since Sisi deposed Mursi. The National Defence Council - comprising the interim president and heads of the security forces - said it was committed to freedom of expression and protest, providing they were peaceful. It urged Mursi supporters to "stop practicing violence and terrorism".

Besides the Cairo bloodshed, some of the worst violence has been in the lawless Sinai peninsula, which borders Israel and the Palestinian Gaza Strip, where Islamist militants have targeted security forces on an almost daily basis.

State news agency MENA said on Sunday that 10 "terrorist elements" in north Sinai had been killed and 20 others arrested in security sweeps over the past 48 hours.

Islamist politician and former presidential candidate Mohamed Selim al-Awa offered a compromise that would see Mursi reinstated but with new elections within months. It was swiftly rejected by the interim presidency and Egypt's biggest liberal and leftist coalition, the National Salvation Front (NSF).

"It's clearly a non-starter," said NSF spokesman Khaled Dawoud. "You can't rewrite history; Mursi is out and there is already a road map."

(Additional reporting by Shadia Nasralla, Yasmine Saleh, Tom Finn, Omar Fahmy, Michael Georgy, Noah Browning, Tom Perry and Asma Alsharif, Patrick Werr in Cairo, Yusri Mohamed in Ismailia; Writing by Crispian Balmer and Matt Robinson; Editing by Michael Georgy and Giles Elgood)

By Maggie Fick and Yasmine Saleh