After sitting politically backstage for the past six months, Egypt's powerful military has been thrust back into the spotlight to handle a crisis that is testing its desire to stay neutral and its duty to maintain order.

The military has called for President Mohamed Morsi and the secular opposition to meet later on Wednesday in a bid to prevent the crisis over an imminent constitutional referendum from tearing the country apart.

It made the call after the Islamist Morsi ordered the army to back up police by protecting "vital state institutions" and giving officers powers to arrest civilians.

After meeting Morsi on Monday, the defence minister and commander of the armed forces, General Abel Fattah al-Sissi, called on army officers to exercise the "highest levels of self-restraint".

He said the armed forces were determined to "carry out their role in protecting the nation and its stability regardless of pressures and challenges".

However, human rights groups see the risk of a return to civilians being tried in military courts, as happened during the transition period between the fall of Hosni Mubarak in February 2011 until Morsi's election in June 2012.

The decree granting the army police powers was slammed by Human Rights Watch, which said it could allow unchecked abuses and that it undermined the rule of law.

Amnesty International called the security decree "a dangerous loophole which may well lead to the military trial of civilians".

Directly or indirectly, the military has had a prominent role in Egyptian life since the overthrow in 1952 of the monarchy.

Morsi is the first civilian to become president. His four predecessors -- Mohammed Naguib, Gamal Abdel Nasser, Anwar Sadat and Mubarak -- all rose from the ranks of the military.

During those years, the army was able to build up a vast economic empire, controlling enterprises from mineral water to property and even cemeteries.

Morsi, despite initially being kept on a tight rein by the generals, succeeded in taking the upper hand in August by forcing Supreme Council of the Armed Forces chief Field Marshal Hussein Tantawi into retirement.

"Under Tantawi's leadership, the military managed the transition period and exercised executive and legislative power for more than a year. It became directly involved in political life," said Amr Rabie of Al-Ahram Institute of Political and Strategic Studies.

"Morsi has worked to restore the military's role as a professional army, but the past few days have shown that it still seeks to play a political role," he added.

On Saturday, the military put its foot down, telling both sides to start dialogue as "only way to reach agreement and achieve the interests of the nation and its citizens".

"The opposite of that will take us into a dark tunnel with disastrous results -- and that is something we will not allow," the army warned.

Should simmering tensions boil over, it remains to be seen whether the military will side with Morsi or the secular opposition.

"It's not clear; it is hard to know what the reaction will be from this institution" which revels in the cult of secrecy, Rabie said.

This puts "enormous pressure on Mr Morsi," as the powerful institution again rises to the fore.

"If there are any violent confrontations and if blood runs in the street then certainly the army will intervene," analyst Emad Gad said.

"Whether the president is elected or not, it is his duty to try to preserve security and keep order."

The risk still exists that the army could roll back to zero the transition begun since Mubarak's ouster, should it again find itself in charge.

"It would then have to preside over yet another transition period, with a new roadmap," said Gad.