The Egyptian army is detaining ousted President Mohammed Morsi over accusations of kidnapping, killing soldiers and other charges, the state news agency said on Friday.

The army had previously said it was holding Morsi for his own safety and the report was likely to stoke tension before mass rallies on Friday billed as shows of strength between supporters and opponents of the Islamist Morsi.

Protesters display posters of Egypt's ousted president Mohammed Morsi during a rally in support of the toppled leader in Kuala Lumpur, Malaysia, on Friday. Lai Seng Sin / Associated Press

Both sides warned of the potential for bloodshed in Egypt, which has been convulsed by political and economic turmoil since the 2011 uprising that ended 30 years of rule by Hosni Mubarak.

State news agency MENA said the mooted charges against Morsi included conspiring with the Palestinian Islamist group Hamas, killing prisoners and officers "deliberately with prior intent", kidnapping officers and soldiers, and setting fire to the prison of Wadi el-Natroun.

They relate to his escape from prison in 2011, when he was arrested during the uprising against Mubarak, and provide legal grounds for his continued detention.

Morsi has been held by the military since the army ousted him from office on July 3 following huge street protests against his troubled, one-year rule. Washington has previously called for him to be freed.

His Muslim Brotherhood denounced news of the accusations.

"At the end of the day we know all of these charges are nothing more than the fantasy of a few army generals and a military dictatorship," Brotherhood spokesman Gehad El-Haddad said. "We are continuing our protests on the streets."

Throwing down the gauntlet to the Brotherhood, army chief General Abdel Fattah al-Sisi has called on Egyptians to rally nationwide on Friday to give the military a "mandate" to confront weeks of violence unleashed by Morsi's removal.

A military official said the army had given the Brotherhood a Saturday deadline to end its resistance and join a military-set road map to new elections, signaling a turning point in the confrontation.

The army has threatened to "turn its guns" on those who use violence. The Brotherhood warned of civil war.

"We will not initiate any move, but will definitely react harshly against any calls for violence or black terrorism from Brotherhood leaders or their supporters," an army official said.

A few hundred pro-army supporters gathered early on Friday morning in Tahrir Square, center of two years of turmoil in Egypt, before the main rally which might not peak until after the evening prayer marking the end of the day's Ramadan fast.