As Mohamed Morsi is detained, Cairo braces for showdown between military and Muslim Brotherhood

Tension was building on the streets of Cairo today as Egyptians awoke anticipating a dangerous last ditch showdown between the military and Muslim Brotherhood. Armoured vehicles have been positioned at key points across the capital, a 48-deadline imposed on Egypt's Islamists is ticking down - and the Brotherhood has warned of "civil war" if the generals launch an expected crackdown this weekend.

In Cairo's Tahrir Square - once a crucible of opposition to Hosni Mubarak and the repressive state he represented - protesters were this morning handing out photos to passing cars featuring the face of General Abdel Fattah al-Sisi, the army commander who startled observers by calling for mass rallies in order to grant his troops a mandate to tackle "terrorism".

He made no mention of the Brotherhood by name. Yet his comments were widely understood to be a grave warning to the Islamist group.

Its leaders - who have implacably rejected the popular coup that ousted Mohamed Morsi earlier this month - have called their own rallies today, raising fears of bloody confrontations. But the military yesterday said the group had until tomorrow to sign up to the ongoing transitional process.

It remains unclear what action Egypt's generals might take. For the past month the Brotherhood has been maintaining an extended sit-in at Rabaa al-Adawiya, a square in eastern Cairo, often with tens of thousands of supporters. If the military sent in tanks to clear the protest, the operation would be a bloodbath.

Today the stakes were raised even higher when a court ordered the detention of Mr Morsi over allegations that he collaborated with Hamas, the Gaza-based Islamist group. The allegations relate to a prison break shortly after the January 2011 revolt in which Mr Morsi and other inmates escaped from jail.

The toppled President is currently being held under detention at an unknown location. But any legal measures would further stoke tensions following a month in which Brotherhood leaders have been targeted by arrest warrants and hundreds of rank and file members rounded up and detained.

So far neither side appears willing to step back from the brink. The Brotherhood's Supreme Guide, Mohamed Badie, yesterday said that the military's role in removing Morsi was a greater crime than destroying the Kaaba, Islam's holiest site. The military, meanwhile, has said it will "turn its guns" on any group which resorts to violence.